RDMA/core: Document CM @event_handler function

Code audit suggests that the RDMA CM event handler callback function is
_always_ invoked in a context that is safe to block. That's important for
consumer implementers to know, so document that in the comment before
rdma_create_id (where the handler function is set up by the consumer).
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

include/rdma/rdma_cm.h

@@ -152,7 +152,11 @@ struct rdma_cm_id *__rdma_create_id(struct net *net,
  * @ps: RDMA port space.
  * @qp_type: type of queue pair associated with the id.
  *
- * The id holds a reference on the network namespace until it is destroyed.
+ * Returns a new rdma_cm_id. The id holds a reference on the network
+ * namespace until it is destroyed.
+ *
+ * The event handler callback serializes on the id's mutex and is
+ * allowed to sleep.
  */
 #define rdma_create_id(net, event_handler, context, ps, qp_type) \
 	__rdma_create_id((net), (event_handler), (context), (ps), (qp_type), \