Documentation/memory-barriers.txt: Prohibit speculative writes

No SMP architecture currently supporting Linux allows
speculative writes, so this commit updates
Documentation/memory-barriers.txt to prohibit them in Linux core
code.  It also records restrictions on their use.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Reviewed-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: <linux-arch@vger.kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/1386799151-2219-3-git-send-email-paulmck@linux.vnet.ibm.com


[ Paul modified the original patch from Peter. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>