docs: mention UNUSED_ALERT_DAYS in alerts docs

See #18292

Author: nijel

docs/devel/alerts.rst

@@ -23,7 +23,7 @@ Currently the following is covered:
 * Ambiguous language code
 * Unused new base in component settings
 * Duplicate file mask used for linked components
-* Component seems unused
+* Component seems unused (configurable by :setting:`UNUSED_ALERT_DAYS`)
 
 The alerts are updated daily, or on related change (for example when
 :ref:`component` is changed or when repository is updated).