Documentation: checkpatch: Add SPLIT_STRING message

Add a new message type SPLIT_STRING under the 'Indentation and Line
Breaks' subsection. Checkpatch documentation for the splitting of
quoted strings that appear in userspace, across multiple lines.

Suggested-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Utkarsh Verma <utkarshverma294@gmail.com>
Link: https://lore.kernel.org/r/20210904073621.12987-1-utkarshverma294@gmail.com


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