[Docs] Improve Target TableGen Docs (#178518)

This change adds documentation for some of the advanced parts of the
TableGen Selection structures:
- The `set`, `node` and `srcvalue` special DAG nodes
- The `SDNodeXForm` C++ snippet.
- The `PatFrags` predicate C++ snippet, and documentation for how the
`OperandTransform` item works in a little more detail.
- The `ImmLeaf` predicate code.

This also adds documentation for the many C++ callbacks for Targets
associated with Decoding, Encoding, Parsing and Printing Operands and
Instructions, including Predicates.

Co-authored-by: Craig Topper <craig.topper@sifive.com>
GitOrigin-RevId: 6f51f8e0f93e6ff2baf0ed2f60021308920a557f