Clarify that authentication is not mandatory in API calls (#20743)

* Clarify that authentication is not mandatory in API calls

* Update auth.rst

* Update docs/topics/api/auth.rst

Co-authored-by: Andrew Williamson <awilliamson@mozilla.com>

---------

Co-authored-by: Andrew Williamson <awilliamson@mozilla.com>

docs/topics/api/auth.rst

@@ -4,7 +4,9 @@
 Authentication (External)
 =========================
 
-To access the API as an external consumer, you need to include a
+To execute authenticated calls against the API as an external consumer, (see the
+documentation for individual endpoints where it will indicate if that endpoint
+requires, or would benefit from, authentication), you need to include a
 `JSON Web Token (JWT)`_ in the ``Authorization`` header for every request.
 This header acts as a one-time token that authenticates your user account.
 No JWT claims are made about the actual API request you are making.