add docstring for ansible.inventory.helpers.get_group_vars

Document the expected parameter type and return type of
get_group_vars().

lib/ansible/inventory/helpers.py

@@ -27,7 +27,12 @@ def sort_groups(groups):
 
 
 def get_group_vars(groups):
+    """
+    Combine all the group vars from a list of inventory groups.
 
+    :param groups: list of ansible.inventory.group.Group objects
+    :rtype: dict
+    """
     results = {}
     for group in sort_groups(groups):
         results = combine_vars(results, group.get_vars())