move comment from header to source

- The logic for relaying RPC breadcrumbs from RPC requests to threads is
  now internal to margo (which is good), just moving comment
  accordingly.

include/margo.h

@@ -990,11 +990,6 @@ hg_return_t _handler_for_##__name(hg_handle_t handle) { \
 /**
  * macro that defines a function to glue an RPC handler to a ult handler
  * @param [in] __name name of handler function
- *
- * Note: we use this opportunity to set a thread-local argobots key that stores
- * the "breadcrumb" that was set in the RPC.  It is shifted down 16 bits so that
- * if this handler in turn issues more RPCs there will be a stack showing their
- * ancestry.
  */
 #define DEFINE_MARGO_RPC_HANDLER(__name) \
     __MARGO_INTERNAL_RPC_WRAPPER(__name) \

src/margo.c

@@ -1958,9 +1958,16 @@ void __margo_internal_pre_wrapper_hooks(margo_instance_id mid, hg_handle_t handl
 {
     hg_return_t ret;
     uint64_t *rpc_breadcrumb;
+
     ret = HG_Get_input_buf(handle, (void**)&rpc_breadcrumb, NULL);
     assert(ret == HG_SUCCESS);
     *rpc_breadcrumb = le64toh(*rpc_breadcrumb);
+    /* Note: we use this opportunity to retrieve the incoming RPC
+     * breadcrumb and put it in a thread-local argobots key.  It is
+     * shifted down 16 bits so that if this handler in turn issues more
+     * RPCs, there will be a stack showing the ancestry of RPC calls that
+     * led to that point.
+     */
     margo_internal_breadcrumb_handler_set((*rpc_breadcrumb) << 16);
 }