Add mappings for selecting syntax nodes within match mode

[llakala]

Three years ago, a note was added to the Keymap documentation, saying TODO: Mappings for selecting syntax nodes (a superset of `[`).

As far as I can tell, these mappings still remain TODO. If they exist and are just undocumented, my mistake.

(Note: there is now an actual workaround to this, via the recently-merged #4709. In fact, this example is referenced in the top of the PR. However, there still should be mappings provided.)