block: mq: Add some minor doc for core structs

As it came up in discussion on the mailing list that the semantic
meaning of 'blk_mq_ctx' and 'blk_mq_hw_ctx' isn't completely
obvious to everyone, let's add some minimal kerneldoc for a
starter.
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq.h

@@ -7,6 +7,9 @@
 
 struct blk_mq_tag_set;
 
+/**
+ * struct blk_mq_ctx - State for a software queue facing the submitting CPUs
+ */
 struct blk_mq_ctx {
 	struct {
 		spinlock_t		lock;

include/linux/blk-mq.h

@@ -9,6 +9,9 @@
 struct blk_mq_tags;
 struct blk_flush_queue;
 
+/**
+ * struct blk_mq_hw_ctx - State for a hardware queue facing the hardware block device
+ */
 struct blk_mq_hw_ctx {
 	struct {
 		spinlock_t		lock;