docs: add documentation for checkpatch

Add documentation for kernel script checkpatch.pl.
This documentation is also parsed by checkpatch to
enable a verbose mode.

The checkpatch message types are grouped by usage. Under
each group the types are described briefly. 34 of such
types are documented.

Signed-off-by: Dwaipayan Ray <dwaipayanray1@gmail.com>
Link: https://lore.kernel.org/r/20210226093827.12700-2-dwaipayanray1@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>