[C++][Docs] Verify Completeness of API Reference

At least the Datum class is missing methods in the API documentation, and any other incompleteness needs to be found and addressed. This ticket is for hunting down incompleteness, and filling in the gaps.

**Reporter**: [Kae Suarez](https://issues.apache.org/jira/browse/ARROW-17186) / @ksuarez1423
#### Related issues:
- [[Docs][C++] Add missing methods to ArrayBuilders API Reference](https://github.com/apache/arrow/issues/32617) (is a parent of)

_{**Note**: *This issue was originally created as [ARROW-17186](https://issues.apache.org/jira/browse/ARROW-17186). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start by comparing the C++ API documentation with the public methods of the Datum class, then check whether other documented APIs have similar gaps. The related ArrayBuilders API Reference issue may help identify the documentation area and expected coverage. Done means all missing API methods are documented and the completeness gaps found by the audit are addressed.

Written by the indexing model from the issue text.