documentation: Distinguish between local and global transitivity

The introduction of smp_load_acquire() and smp_store_release() had
the side effect of introducing a weaker notion of transitivity:
The transitivity of full smp_mb() barriers is global, but that
of smp_store_release()/smp_load_acquire() chains is local.  This
commit therefore introduces the notion of local transitivity and
gives an example.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Reported-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>