tools/memory-model: Expand the cheatsheet.txt notion of relaxed

This commit adds a key entry enumerating the various types of relaxed
operations.  While in the area, it also renames the relaxed rows.

[ paulmck: Apply Boqun Feng feedback. ]
Acked-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>