docs: Add more message type documentations for checkpatch

- Document a couple of more checkpatch message types.
- Add a blank line before all `See:` lines to improve the
  rst output.
- Create a new subsection `Permissions` and move a few types
  to it.

Signed-off-by: Dwaipayan Ray <dwaipayanray1@gmail.com>
Acked-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20210515132348.19082-1-dwaipayanray1@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>