MAINTAINERS: new Documentation maintainer

Transfer Documentation maintainership to Jiri Kosina.
Thanks, Jiri.

I'll still be reviewing and working on documentation.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>