docs: checkpatch: Document and segregate more checkpatch message types

Add and document more checkpatch message types. About 50% of all
message types are documented now.

In addition to this:

- Create a new subsection 'Indentation and Line Breaks'.
- Rename subsection 'Comment style' to simply 'Comments'.
- Refactor some of the existing types to appropriate subsections.
Reviewed-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Tested-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Dwaipayan Ray <dwaipayanray1@gmail.com>
Link: https://lore.kernel.org/r/20210614141132.6881-1-dwaipayanray1@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>