docs: checkpatch: add multiline, do/while, and multiple-assignment messages

Added and documented 3 new message types:
- MULTILINE_DEREFERENCE
- SINGLE_STATEMENT_DO_WHILE_MACRO
- MULTIPLE_ASSIGNMENTS

Signed-off-by: Utkarsh Verma <utkarshverma294@gmail.com>
Link: https://lore.kernel.org/r/20210925163852.12096-1-utkarshverma294@gmail.com


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