Documentation/memory-barriers.txt: Prohibit speculative writes

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 <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
Reviewed-by: Josh Triplett <[email protected]>
Reviewed-by: Peter Zijlstra <[email protected]>
Cc: <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Andrew Morton <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
[ Paul modified the original patch from Peter. ]
Signed-off-by: Ingo Molnar <[email protected]>