documentation: Remove obsolete reference to RCU-protected indexes

Commit #1ebee8017d (rcu: Eliminate array-index-based RCU primitives)
eliminated the primitives supporting RCU-protected array indexes, but
failed to update Documentation/memory-barriers.txt accordingly.  This
commit therefore removes the discussion of RCU-protected array indexes.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney 2016-01-14 14:01:34 -08:00
commit 0e4bd2aba3