documentation: Add alternative release-acquire outcome

documentation:  Add alternative release-acquire outcome

The memory-barriers.txt discussion of local transitivity and
release-acquire chains leaves out discussion of the outcome of
the read from "u".  This commit therefore adds an outcome showing
that you can get a "1" from this read even if the release-acquire
pairs don't line up.

Reported-by: Will Deacon <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>