[DOCS] SQL: Document null handling for functions (#74444)

Closes #74193.

docs/reference/sql/functions/aggs.asciidoc

@@ -21,7 +21,8 @@ AVG(numeric_field) <1>
 
 *Input*:
 
-<1> numeric field
+<1> numeric field. If this field contains only `null` values, the function
+returns `null`. Otherwise, the function ignores `null` values in this field.
 
 *Output*: `double` numeric value
 
@@ -48,16 +49,14 @@ COUNT(expression) <1>
 
 *Input*:
 
-<1> a field name, wildcard (`*`) or any numeric value
+<1> a field name, wildcard (`*`) or any numeric value. For `COUNT(*)` or
+`COUNT(<literal>)`, all values are considered, including `null` or missing
+ones. For `COUNT(<field_name>)`, `null` values are not considered.
 
 *Output*: numeric value
 
 *Description*: Returns the total number (count) of input values.
 
-In case of `COUNT(*)` or `COUNT(<literal>)`, _all_ values are considered (including `null` or missing ones).
-
-In case of `COUNT(<field_name>)` `null` values are not considered.
-
 
 ["source","sql",subs="attributes,macros"]
 --------------------------------------------------
@@ -76,7 +75,8 @@ COUNT(ALL field_name) <1>
 
 *Input*:
 
-<1> a field name
+<1> a field name. If this field contains only `null` values, the function
+returns `null`. Otherwise, the function ignores `null` values in this field.
 
 *Output*: numeric value
 
@@ -105,7 +105,8 @@ COUNT(DISTINCT field_name) <1>
 
 <1> a field name
 
-*Output*: numeric value
+*Output*: numeric value. If this field contains only `null` values, the function
+returns `null`. Otherwise, the function ignores `null` values in this field.
 
 *Description*: Returns the total number of _distinct non-null_ values in input values.
 
@@ -137,7 +138,7 @@ FIRST(
 
 *Output*: same type as the input
 
-*Description*: Returns the first **non-NULL** value (if such exists) of the `field_name` input column sorted by
+*Description*: Returns the first non-`null` value (if such exists) of the `field_name` input column sorted by
 the `ordering_field_name` column. If `ordering_field_name` is not provided, only the `field_name`
 column is used for the sorting. E.g.:
 
@@ -237,7 +238,7 @@ LAST(
 
 *Output*: same type as the input
 
-*Description*: It's the inverse of <<sql-functions-aggs-first>>. Returns the last **non-NULL** value (if such exists) of the
+*Description*: It's the inverse of <<sql-functions-aggs-first>>. Returns the last non-`null` value (if such exists) of the
 `field_name` input column sorted descending by the `ordering_field_name` column. If `ordering_field_name` is not
 provided, only the `field_name` column is used for the sorting. E.g.:
 
@@ -330,7 +331,8 @@ MAX(field_name) <1>
 
 *Input*:
 
-<1> a numeric field
+<1> a numeric field. If this field contains only `null` values, the function
+returns `null`. Otherwise, the function ignores `null` values in this field.
 
 *Output*: same type as the input
 
@@ -361,7 +363,8 @@ MIN(field_name) <1>
 
 *Input*:
 
-<1> a numeric field
+<1> a numeric field. If this field contains only `null` values, the function
+returns `null`. Otherwise, the function ignores `null` values in this field.
 
 *Output*: same type as the input
 
@@ -387,7 +390,8 @@ SUM(field_name) <1>
 
 *Input*:
 
-<1> a numeric field
+<1> a numeric field. If this field contains only `null` values, the function
+returns `null`. Otherwise, the function ignores `null` values in this field.
 
 *Output*: `bigint` for integer input, `double` for floating points
 
@@ -418,7 +422,8 @@ KURTOSIS(field_name) <1>
 
 *Input*:
 
-<1> a numeric field
+<1> a numeric field. If this field contains only `null` values, the function
+returns `null`. Otherwise, the function ignores `null` values in this field.
 
 *Output*: `double` numeric value
 
@@ -452,7 +457,8 @@ MAD(field_name) <1>
 
 *Input*:
 
-<1> a numeric field
+<1> a numeric field. If this field contains only `null` values, the function
+returns `null`. Otherwise, the function ignores `null` values in this field.
 
 *Output*: `double` numeric value
 
@@ -485,8 +491,10 @@ PERCENTILE(
 
 *Input*:
 
-<1> a numeric field
-<2> a numeric expression (must be a constant and not based on a field)
+<1> a numeric field. If this field contains only `null` values, the function
+returns `null`. Otherwise, the function ignores `null` values in this field.
+<2> a numeric expression (must be a constant and not based on a field). If
+`null`, the function returns `null`.
 <3> optional string literal for the <<search-aggregations-metrics-percentile-aggregation-approximation,percentile algorithm>>. Possible values: `tdigest` or `hdr`. Defaults to `tdigest`.
 <4> optional numeric literal that configures the <<search-aggregations-metrics-percentile-aggregation-approximation,percentile algorithm>>. Configures `compression` for `tdigest` or `number_of_significant_value_digits` for `hdr`. The default is the same as that of the backing algorithm.
 
@@ -527,8 +535,10 @@ PERCENTILE_RANK(
 
 *Input*:
 
-<1> a numeric field
-<2> a numeric expression (must be a constant and not based on a field)
+<1> a numeric field. If this field contains only `null` values, the function
+returns `null`. Otherwise, the function ignores `null` values in this field.
+<2> a numeric expression (must be a constant and not based on a field). If
+`null`, the function returns `null`.
 <3> optional string literal for the <<search-aggregations-metrics-percentile-aggregation-approximation,percentile algorithm>>. Possible values: `tdigest` or `hdr`. Defaults to `tdigest`.
 <4> optional numeric literal that configures the <<search-aggregations-metrics-percentile-aggregation-approximation,percentile algorithm>>. Configures `compression` for `tdigest` or `number_of_significant_value_digits` for `hdr`. The default is the same as that of the backing algorithm.
 
@@ -566,7 +576,8 @@ SKEWNESS(field_name) <1>
 
 *Input*:
 
-<1> a numeric field
+<1> a numeric field. If this field contains only `null` values, the function
+returns `null`. Otherwise, the function ignores `null` values in this field.
 
 *Output*: `double` numeric value
 
@@ -600,7 +611,8 @@ STDDEV_POP(field_name) <1>
 
 *Input*:
 
-<1> a numeric field
+<1> a numeric field. If this field contains only `null` values, the function
+returns `null`. Otherwise, the function ignores `null` values in this field.
 
 *Output*: `double` numeric value
 
@@ -629,7 +641,8 @@ STDDEV_SAMP(field_name) <1>
 
 *Input*:
 
-<1> a numeric field
+<1> a numeric field. If this field contains only `null` values, the function
+returns `null`. Otherwise, the function ignores `null` values in this field.
 
 *Output*: `double` numeric value
 
@@ -658,7 +671,8 @@ SUM_OF_SQUARES(field_name) <1>
 
 *Input*:
 
-<1> a numeric field
+<1> a numeric field. If this field contains only `null` values, the function
+returns `null`. Otherwise, the function ignores `null` values in this field.
 
 *Output*: `double` numeric value
 
@@ -687,7 +701,8 @@ VAR_POP(field_name) <1>
 
 *Input*:
 
-<1> a numeric field
+<1> a numeric field. If this field contains only `null` values, the function
+returns `null`. Otherwise, the function ignores `null` values in this field.
 
 *Output*: `double` numeric value
 
@@ -717,7 +732,8 @@ VAR_SAMP(field_name) <1>
 
 *Input*:
 
-<1> a numeric field
+<1> a numeric field. If this field contains only `null` values, the function
+returns `null`. Otherwise, the function ignores `null` values in this field.
 
 *Output*: `double` numeric value