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>