documentation: explain memory barriers

We want all uses of memory barriers to be explained in the source code.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/SubmitChecklist

@@ -85,3 +85,6 @@ kernel patches.
 23: Tested after it has been merged into the -mm patchset to make sure
     that it still works with all of the other queued patches and various
     changes in the VM, VFS, and other subsystems.
+
+24: All memory barriers {e.g., barrier(), rmb(), wmb()} need a comment in the
+    source code that explains the logic of what they are doing and why.