[PR #9040/cc72fa07 backport][stable-9] groupby_as_dict: mention Jinja2's groupby filter (#9043)

groupby_as_dict: mention Jinja2's groupby filter (#9040)

Mention Jinja2's groupby filter.

(cherry picked from commit cc72fa0786)

Co-authored-by: Felix Fontein <felix@fontein.de>

plugins/filter/groupby_as_dict.py

@@ -13,6 +13,8 @@ DOCUMENTATION = '''
   author: Felix Fontein (@felixfontein)
   description:
     - Transform a sequence of dictionaries to a dictionary where the dictionaries are indexed by an attribute.
+    - This filter is similar to the Jinja2 C(groupby) filter. Use the Jinja2 C(groupby) filter if you have multiple entries with the same value,
+      or when you need a dictionary with list values, or when you need to use deeply nested attributes.
   positional: attribute
   options:
     _input: