Documented the settings variable for WAGTAIL_USAGE_COUNT_ENABLED (#2706)

docs/advanced_topics/settings.rst

@@ -332,6 +332,18 @@ a custom user model is being used and extra fields are required in the user crea
 
 A list of the extra custom fields to be appended to the default list.
 
+Usage for images, documents and snippets
+----------------------------------------
+
+.. code-block:: python
+
+    WAGTAIL_USAGE_COUNT_ENABLED = True
+
+When enabled Wagtail shows where a particular image, document or snippet is being used on your site (disabled by default). A link will appear on the edit page showing you which pages they have been used on.
+
+.. note::
+
+    The usage count only applies to direct (database) references. Using documents, images and snippets within StreamFields or rich text fields will not be taken into account.
 
 URL Patterns
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