[PATCH] update kfree, vfree, and vunmap kerneldoc

This patch clarifies NULL handling of kfree() and vfree().  I addition,
wording of calling context restriction for vfree() and vunmap() are changed
from "may not" to "must not."

Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Acked-by: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>