ipc: update locking scheme comments

The initial documentation was a bit incomplete, update accordingly.

[akpm@linux-foundation.org: make it more readable in 80 columns]
Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
Acked-by: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>