add an example of indexing related model fields

docs/core_components/search/indexing.rst

@@ -154,6 +154,21 @@ One use for this is indexing ``get_*_display`` methods Django creates automatica
             index.FilterField('is_private'),
         )
 
+Callables also provide a way to index fields from related models. In the example from :ref:`inline_panels`, to index each BookPage by the titles of its related_links:
+
+.. code-block:: python
+
+    class BookPage(Page):
+    ...
+
+    def get_related_link_titles(self):
+        # Get list of titles and concatenate them
+        return '\n'.join(self.related_links.values_list('text', flat=True))
+
+    search_fields = Page.search_fields + [
+        ...
+        index.SearchField('get_related_link_titles'),
+    ]
 
 .. _wagtailsearch_indexing_models: