Mention nitpick_ignore(_regex) in docs of warn-missing-reference

doc/extdev/appapi.rst

@@ -298,7 +298,9 @@ Here is a more detailed list of these events.
 
    Emitted when a cross-reference to an object cannot be resolved even after
    :event:`missing-reference`.  If the event handler can emit warnings for
-   the missing reference, it should return ``True``.
+   the missing reference, it should return ``True``. The configuration variables
+   :confval:`nitpick_ignore` and :confval:`nitpick_ignore_regex` prevent the
+   event from being emitted for the corresponding nodes.
 
    .. versionadded:: 3.4