Clarify priority setting in Inventory doc (#51343)

* priority setting note in inventory doc

The `ansible_group_priority` variable is also described in *Using Variables* and was given a clarification in issue #41512. The variable is also documented in *Working with Inventory* without the caveat that it needs to be declared on the inventory and not a group_vars file. This change adds that as a note

+label: docsite_pr

docs/docsite/rst/user_guide/intro_inventory.rst

@@ -438,6 +438,7 @@ Starting in Ansible version 2.4, users can use the group variable ``ansible_grou
 
 In this example, if both groups have the same priority, the result would normally have been ``testvar == b``, but since we are giving the ``a_group`` a higher priority the result will be ``testvar == a``.
 
+.. note:: ``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.
 
 .. _using_multiple_inventory_sources: