kdump: documentation cleanups

This cleans up kdump documentation a bit. Plus I do not think we want
to mention Linux trademark in _every_ file in documentation....

Signed-off-by: Pavel Machek <pavel@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>