tools/memory-model: docs/README: Update introduction of locking.txt

tools/memory-model: docs/README: Update introduction of locking.txt

Commit 9bc931e9e161 ("tools/memory-model: Add locking.txt and
glossary.txt to README") failed to mention the relation of the "Locking"
section in recipes.txt and locking.txt.

The latter is a detailed version of the former intended to be read on
its own.

Reword the description in README and add notes in locking.txt and
recipes.txt to clarify their relationship.

[ paulmck: Wordsmithing. ]

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Acked-by: Andrea Parri <parri.andrea@gmail.com>