[Docs][Format] Make Variant value field mandatory

[sdf-jkl]

Describe the enhancement requested

Fix wording in the Arrow Variant spec to make the value field mandatory.

arrow/docs/source/format/CanonicalExtensions.rst

Line 448 in f10c93c

* At least one (or both) of the following:

Per the conversation in apache/parquet-format#591 (comment)

Note the requirement is level-dependent in the Parquet spec, so the Arrow wording should mirror it level by level rather than making value mandatory everywhere:

• top-level variant group: value must be present
• shredded object fields: the field group contains both value and typed_value
• list elements: value may be omitted when shredding elements as a specific type, but at least one of the two fields must be present

Component(s)

Format

[LefterisXefteris]

hello @sdf-jkl can i work on it?

[sdf-jkl]

Hi @LefterisXefteris, go for it!