xprtrdma: Add documenting comment for rpcrdma_buffer_destroy

Make a note of the function's dependency on an earlier ib_drain_qp.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/xprtrdma/verbs.c

@@ -1177,6 +1177,14 @@ rpcrdma_mrs_destroy(struct rpcrdma_buffer *buf)
 	dprintk("RPC:       %s: released %u MRs\n", __func__, count);
 }
 
+/**
+ * rpcrdma_buffer_destroy - Release all hw resources
+ * @buf: root control block for resources
+ *
+ * ORDERING: relies on a prior ib_drain_qp :
+ * - No more Send or Receive completions can occur
+ * - All MRs, reps, and reqs are returned to their free lists
+ */
 void
 rpcrdma_buffer_destroy(struct rpcrdma_buffer *buf)
 {