Address comments

format/spec.md

@@ -178,7 +178,10 @@ A **`list`** is a collection of values with some element type. The element field
 
 A **`map`** is a collection of key-value pairs with a key type and a value type. Both the key field and value field each have an integer id that is unique in the table schema. Map keys are required and map values can be either optional or required. Both map keys and map values may be any type, including nested types.
 
-A **`variant`** is a type to represent semi-structured data. A variant value can store a value of any other type, including any primitive, struct, list or map values. The variant value is encoded in its own binary [encoding](https://github.com/apache/parquet-format/blob/master/VariantEncoding.md). Variant type is added in [v3](#version-3).
+### Semi-structured Types
+
+A **`variant`** is a type to represent semi-structured data. A variant value can store a value of any other type, including any primitive, struct, list or map value. The variant encoding is defined the [Apache Parquet Project](https://github.com/apache/parquet-format/blob/4f208158dba80ff4bff4afaa4441d7270103dff6/VariantEncoding.md). Variant type is added in [v3](#version-3).
+Limitation: only map value with string-type keys is supported in variant.
 
 ### Primitive Types
 
@@ -1352,14 +1355,13 @@ This serialization scheme is for storing single values as individual binary valu
 ## Appendix E: Format version changes
 
 ### Version 3
-New type `variant` is added in V3.
 
 Default values are added to struct fields in v3.
 
 * The `write-default` is a forward-compatible change because it is only used at write time. Old writers will fail because the field is missing.
 * Tables with `initial-default` will be read correctly by older readers if `initial-default` is always null for optional fields. Otherwise, old readers will default optional columns with null. Old readers will fail to read required fields which are populated by `initial-default` because that default is not supported.
 
-Types `unknown`, `timestamp_ns`, and `timestamptz_ns` are added in v3.
+Types `variant`, `unknown`, `timestamp_ns`, and `timestamptz_ns` are added in v3.
 
 All readers are required to read tables with unknown partition transforms, ignoring the unsupported partition fields when filtering.