Extends 'which notation should I use' section in FAQ (#43729)

* Extends 'which notation should I use' section in FAQ

docs/docsite/rst/reference_appendices/faq.rst

@@ -452,8 +452,10 @@ Ansible supports dot notation and array notation for variables. Which notation s
 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 
 The dot notation comes from Jinja and works fine for variables without special
-characters. If your variable contains dots (.), colons (:), or dashes (-) it is
+characters. If your variable contains dots (.), colons (:), or dashes (-), if
-safer to use the array notation for variables.
+a key begins and ends with two underscores, or if a key uses any of the known
+public attributes, it is safer to use the array notation. See :ref:`playbooks_variables`
+for a list of the known public attributes.
 
 .. code-block:: jinja