[3.0.x] Fixed #31126 -- Doc'd STATICFILES_DIRS namespacing in static files how-to.

Backport of a45c8d7ad04b73e33b6989c3ffa2b8c51ae3e83b from master

docs/howto/static-files/index.txt

@@ -71,6 +71,9 @@ details on how ``staticfiles`` finds your files.
     by putting those static files inside *another* directory named for the
     application itself.
 
+    You can namespace static assets in :setting:`STATICFILES_DIRS` by
+    specifying :ref:`prefixes <staticfiles-dirs-prefixes>`.
+
 .. _serving-static-files-in-development:
 
 Serving static files during development

docs/ref/settings.txt

@@ -3324,6 +3324,8 @@ your additional files directory(ies) e.g.::
 Note that these paths should use Unix-style forward slashes, even on Windows
 (e.g. ``"C:/Users/user/mysite/extra_static_content"``).
 
+.. _staticfiles-dirs-prefixes:
+
 Prefixes (optional)
 ~~~~~~~~~~~~~~~~~~~