UCP: Fix comment

src/ucp/rndv/rndv_put.c

@@ -572,11 +572,12 @@ ucp_proto_rndv_put_mtype_probe(const ucp_proto_init_params_t *init_params)
  ucs_memory_type_t frag_mem_type;
 
  if (init_params->rkey_config_key == NULL) {
- /* FIXME: maybe can initialize proto with all available types if no
- * rkey in RTR.
- */
  frag_mem_type = UCS_MEMORY_TYPE_HOST;
  } else {
+ /* Can initialize only the same fragment type as received in RTR
+ * because pipeline protocols assume that both peers use the same
+ * fragment sizes (and they are different for different memory types by
+ * default). */
  frag_mem_type = init_params->rkey_config_key->mem_type;
  }