Documentation: checkpatch: Add SYMBOLIC_PERMS message

Add a new message type SYMBOLIC_PERMS under the 'Permissions'
subsection. Octal permission bits are easier to read and understand
instead of their symbolic macro names.

Suggested-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Utkarsh Verma <utkarshverma294@gmail.com>
Reviewed-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Acked-by: Dwaipayan Ray <dwaipayanray1@gmail.com>
Link: https://lore.kernel.org/r/20210904082330.14864-1-utkarshverma294@gmail.com


[jc: Tweaked wording as suggested by Dwaipayan Ray]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>