clarify priority setting (#44865)

* clarify priority setting

* updated as per fb

docs/docsite/rst/user_guide/playbooks_variables.rst

@@ -866,6 +866,7 @@ Basically, anything that goes into "role defaults" (the defaults folder inside t
 .. note:: The previous describes the default config ``hash_behaviour=replace``, switch to ``merge`` to only partially overwrite.
 .. note:: Group loading follows parent/child relationships. Groups of the same 'parent/child' level are then merged following alphabetical order.
           This last one can be superceeded by the user via ``ansible_group_priority``, which defaults to ``1`` for all groups.
+          This variable, ``ansible_group_priority``, can only be set in the inventory source and not in group_vars/ as the variable is used in the loading of group_vars/.
 
 
 Another important thing to consider (for all versions) is that connection variables override config, command line and play/role/task specific options and keywords.  For example::