MAINTAINERS: resume as Documentation maintainer

I am the new kernel tree Documentation maintainer (except for parts that
are handled by other people, of course).

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Rob Landley <rob@landley.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>