[C++] [Documentation] add discussion of Union.typeIds to Layout.rst

Union.typeIds is poorly documented and the corresponding property in UnionType is confusingly named type_codes. In particular, Layout.rst doesn't include an explanation of Union.typeIds and implies that an element of a union array's type_ids buffer is always the index of a child array.

**Reporter**: [Ben Kietzman](https://issues.apache.org/jira/browse/ARROW-5933) / @bkietz

_{**Note**: *This issue was originally created as [ARROW-5933](https://issues.apache.org/jira/browse/ARROW-5933). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}