Documentation/memory-barriers.txt: various fixes

Fix various grammatical issues in Documentation/memory-barriers.txt.

Cc: "Robert P. J. Day" <rpjday@mindspring.com>
Signed-off-by: Jarek Poplawski <jarkao2@o2.pl>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>