docs: document all error message types in checkpatch

All the error message types now have a verbose description.

Also there are two new groups of message types:

- Macros, Attributes and Symbols
- Functions and Variables

Rearrange the message types to fit these new groups as needed.
Signed-off-by: Dwaipayan Ray <dwaipayanray1@gmail.com>
Reviewed-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20210322082139.33822-1-dwaipayanray1@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>