[PATCH] Update some VFS documentation

Flesh out the description of the address_space operations.
Signed-off-by: Neil Brown <neilb@suse.de>
Cc: Avishay Traeger <atraeger@cs.sunysb.edu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>