Document register_page_listing_more_buttons (#3377)

* add docs for register_page_listing_more_buttons

docs/reference/hooks.rst

@@ -558,6 +558,30 @@ Page explorer
   The ``priority`` argument controls the order the buttons are displayed in. Buttons are ordered from low to high priority, so a button with ``priority=10`` will be displayed before a button with ``priority=20``.
 
 
+.. register_page_listing_more_buttons:
+
+``register_page_listing_more_buttons``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+  Add buttons to the "More" dropdown menu for a page in the page explorer. This works similarly to the ``register_page_listing_buttons`` hook but is useful for lesser-used custom actions that are better suited for the dropdown.
+  
+  This example will add a simple button to the dropdown menu:
+
+  .. code-block:: python
+
+    from wagtail.wagtailadmin import widgets as wagtailadmin_widgets
+
+    @hooks.register('register_page_listing_more_buttons')
+    def page_listing_more_buttons(page, page_perms, is_parent=False):
+        yield wagtailadmin_widgets.PageListingButton(
+            'A dropdown button',
+            '/goes/to/a/url/',
+            priority=10
+        )
+
+  The ``priority`` argument controls the order the buttons are displayed in the dropdown. Buttons are ordered from low to high priority, so a button with ``priority=10`` will be displayed before a button with ``priority=20``.
+
+
 Buttons with dropdown lists
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^