tools/memory-model: Document locking corner cases

Most Linux-kernel uses of locking are straightforward, but there are
corner-case uses that rely on less well-known aspects of the lock and
unlock primitives.  This commit therefore adds a locking.txt and litmus
tests in Documentation/litmus-tests/locking to explain these corner-case
uses.

[ paulmck: Apply Andrea Parri feedback for klitmus7. ]
[ paulmck: Apply Akira Yokosawa example-consistency feedback. ]

Reviewed-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>