docs: move locking-specific documents to locking/

Several files under Documentation/*.txt describe some type of
locking API. Move them to locking/ subdir and add to the
locking/index.rst index file.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/dd833a10bbd0b2c1461d78913f5ec28a7e27f00b.1588345503.git.mchehab+huawei@kernel.org


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