Mention that Python keywords are invalid Ansible variable names (#65094)

* Mention that Python keywords are invalid Ansible variable names
Using a Python keyword as a variable name triggers the error `Invalid variable name in 'register' specified: 'return'`.

docs/docsite/rst/user_guide/playbooks_variables.rst

@@ -30,6 +30,8 @@ Variable names should be letters, numbers, and underscores.  Variables should al
 
 ``foo-port``, ``foo port``, ``foo.port`` and ``12`` are not valid variable names.
 
+`Python keywords <https://docs.python.org/3/reference/lexical_analysis.html#keywords>`_  such as ``async`` and ``lambda`` are not valid variable names and thus must be avoided.
+
 YAML also supports dictionaries which map keys to values.  For instance::
 
   foo: