MAINTAINERS: add entries for block layer documentation

Include Documentation/block/ and Documentation/ABI/stable/sysfs-block in
the "BLOCK LAYER" maintainers file entry.
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20211209003833.6396-9-ebiggers@kernel.orgSigned-off-by: Jens Axboe <axboe@kernel.dk>

MAINTAINERS

@@ -3380,6 +3380,8 @@ M:	Jens Axboe <axboe@kernel.dk>
 L:	linux-block@vger.kernel.org
 S:	Maintained
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/axboe/linux-block.git
+F:	Documentation/ABI/stable/sysfs-block
+F:	Documentation/block/
 F:	block/
 F:	drivers/block/
 F:	include/linux/blk*