Doc: playbook keywords are reserved names too (#69585)

docs/docsite/rst/user_guide/playbooks_variables.rst

@@ -18,7 +18,7 @@ Once you understand the concepts and examples on this page, read about :ref:`Ans
 Creating valid variable names
 =============================
 
-Not all strings are valid Ansible variable names. A variable name must start with a letter, and can only include letters, numbers, and underscores. `Python keywords`_ are not valid variable names.
+Not all strings are valid Ansible variable names. A variable name must start with a letter, and can only include letters, numbers, and underscores. `Python keywords`_ and :ref:`playbook keywords<playbook_keywords>` are not valid variable names.
 
 .. table::
    :class: documentation-table
@@ -28,6 +28,8 @@ Not all strings are valid Ansible variable names. A variable name must start wit
    ====================== ====================================================================
    ``foo``                ``*foo``, `Python keywords`_ such as ``async`` and ``lambda``
 
+   ``foo_env``            :ref:`playbook keywords<playbook_keywords>` such as ``environment``
+
    ``foo_port``           ``foo-port``, ``foo port``, ``foo.port``
 
    ``foo5``               ``5foo``, ``12``