[Canvas] Update function reference docs (elastic#156409)

## Summary

Closes elastic#141526.

This updates the function reference docs for Canvas expressions. The
main update here is the deprecation message added to `filters` function.
I'm also removing an expression example that doesn't work.

<img width="866" alt="Screenshot 2023-05-02 at 9 26 54 AM"
src="https://user-images.githubusercontent.com/1697105/235727031-8ece820e-3db5-42c7-ba11-e94d647a8b84.png">

@KOTungseth These changes impact as early as v8.4. Should I backport all
the way back through 8.4?

### Checklist

Delete any items that are not applicable to this PR.

- [ ] Any text added follows [EUI's writing
guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses
sentence case text and includes [i18n
support](https://github.com/elastic/kibana/blob/main/packages/kbn-i18n/README.md)
- [ ]
[Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html)
was added for features that require explanation or tutorials
- [ ] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios
- [ ] Any UI touched in this PR is usable by keyboard only (learn more
about [keyboard accessibility](https://webaim.org/techniques/keyboard/))
- [ ] Any UI touched in this PR does not create any new axe failures
(run axe in browser:
[FF](https://addons.mozilla.org/en-US/firefox/addon/axe-devtools/),
[Chrome](https://chrome.google.com/webstore/detail/axe-web-accessibility-tes/lhdoppojpmngadmnindnejefpokejbdd?hl=en-US))
- [ ] If a plugin configuration key changed, check if it needs to be
allowlisted in the cloud and added to the [docker
list](https://github.com/elastic/kibana/blob/main/src/dev/build/tasks/os_packages/docker_generator/resources/base/bin/kibana-docker)
- [ ] This renders correctly on smaller devices using a responsive
layout. (You can test this [in your
browser](https://www.browserstack.com/guide/responsive-testing-on-local-server))
- [ ] This was checked for [cross-browser
compatibility](https://www.elastic.co/support/matrix#matrix_browsers)

### Risk Matrix

Delete this section if it is not applicable to this PR.

Before closing this PR, invite QA, stakeholders, and other developers to
identify risks that should be tested prior to the change/feature
release.

When forming the risk matrix, consider some of the following examples
and how they may potentially impact the change:

| Risk | Probability | Severity | Mitigation/Notes |

|---------------------------|-------------|----------|-------------------------|
| Multiple Spaces&mdash;unexpected behavior in non-default Kibana Space.
| Low | High | Integration tests will verify that all features are still
supported in non-default Kibana Space and when user switches between
spaces. |
| Multiple nodes&mdash;Elasticsearch polling might have race conditions
when multiple Kibana nodes are polling for the same tasks. | High | Low
| Tasks are idempotent, so executing them multiple times will not result
in logical error, but will degrade performance. To test for this case we
add plenty of unit tests around this logic and document manual testing
procedure. |
| Code should gracefully handle cases when feature X or plugin Y are
disabled. | Medium | High | Unit tests will verify that any feature flag
or plugin combination still results in our service operational. |
| [See more potential risk
examples](https://github.com/elastic/kibana/blob/main/RISK_MATRIX.mdx) |

### For maintainers

- [ ] This was checked for breaking API changes and was [labeled
appropriately](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

---------

Co-authored-by: Kaarina Tungseth <[email protected]>
(cherry picked from commit b5e8ab7)

docs/canvas/canvas-function-reference.asciidoc

@@ -1018,6 +1018,8 @@ Aliases: `q`, `query`
 Default: `"-_index:.kibana"`
 
 |`index`
+
+Alias: `dataView`
 |`string`
 |An index or data view. For example, `"logstash-*"`.
 
@@ -1087,6 +1089,8 @@ Default: `1000`
 |A comma-separated list of fields. For better performance, use fewer fields.
 
 |`index`
+
+Alias: `dataView`
 |`string`
 |An index or data view. For example, `"logstash-*"`.
 
@@ -1128,7 +1132,7 @@ kibana
 ----
 This retrieves the `Carrier`, `FlightDelayMin`, and `AvgTicketPrice` fields from the "kibana_sample_data_flights" index.
 
-*Accepts:* `filter`
+*Accepts:* `kibana_context`, `null`
 
 [cols="3*^<"]
 |===
@@ -1152,6 +1156,12 @@ Alias: `param`
 |`string`, `number`, `boolean`
 |A parameter to be passed to the SQL query.
 
+|`timeField`
+
+Alias: `timeField`
+|`string`
+|The time field to use in the time range filter, which is set in the context.
+
 |`timezone`
 
 Alias: `tz`
@@ -1178,19 +1188,6 @@ exactly "age" value=50 filterGroup="group2"
 exactly column="project" value="beats"
 ----
 
-*Code example*
-[source,text]
-----
-kibana
-| selectFilter
-| exactly column=project value=elasticsearch
-| demodata
-| pointseries x=project y="mean(age)"
-| plot defaultStyle={seriesStyle bars=1}
-| render
-----
-The `exactly` filter here is added to existing filters retrieved by the `filters` function and further filters down the data to only have `"elasticsearch"` data. The `exactly` filter only applies to this one specific element and will not affect other elements in the workpad.
-
 *Accepts:* `filter`
 
 [cols="3*^<"]
@@ -1270,7 +1267,7 @@ Aliases: `exp`, `expression`, `fn`, `function`
 [[filters_fn]]
 === `filters`
 
-Aggregates element filters from the workpad for use elsewhere, usually a data source.
+Aggregates element filters from the workpad for use elsewhere, usually a data source. <<filters_fn>> is deprecated and will be removed in a future release. Use `kibana | selectFilter` instead.
 
 *Expression syntax*
 [source,js]
@@ -1935,7 +1932,7 @@ Alias: `expression`
 [[mathColumn_fn]]
 === `mathColumn`
 
-Adds a column by evaluating TinyMath on each row. This function is optimized for math, so it performs better than the mapColumn with a math
+Adds a column by evaluating `TinyMath` on each row. This function is optimized for math and performs better than using a math expression in <<mapColumn_fn>>.
 
 *Accepts:* `datatable`
 
@@ -1955,6 +1952,10 @@ Alias: `expression`
 |`string`
 |An evaluated `TinyMath` expression. See https://www.elastic.co/guide/en/kibana/current/canvas-tinymath-functions.html.
 
+|`castColumns` †
+|`string`
+|The column ids that are cast to numbers before the formula is applied.
+
 |`copyMetaFrom`
 |`string`, `null`
 |If set, the meta object from the specified column id is copied over to the specified target column. If the column doesn't exist it silently fails.