Clarify documentation on customizing the ignored pattern list for `collectstatic`

[Tobias Kunze]

The docs (​https://docs.djangoproject.com/en/1.10/ref/contrib/staticfiles/#customizing-the-ignored-pattern-list) only include two very terse sentences on the new feature of customizing the ignored pattern list for collectstatic via code instead of invocation parameters. The second sentence ends with

override the ignore_patterns attribute of this class and specify that class path inside your INSTALLED_APPS setting.

If you just follow those instructions without removing 'staticfiles' from the INSTALLED_APPS list, you'll get the error Application labels aren't unique, duplicates: staticfiles.

[Tim Graham]

Is this enough?

docs/ref/contrib/staticfiles.txt

@@ -136 +136 @@
 The default ignored pattern list, ``['CVS', '.*', '*~']``, can be customized in
 a more persistent way than providing the ``--ignore`` command option at each
 ``collectstatic`` invocation. Provide a custom :class:`~django.apps.AppConfig`
-class, override the ``ignore_patterns`` attribute of this class and specify
-that class path inside your :setting:`INSTALLED_APPS` setting:
+class, override the ``ignore_patterns`` attribute of this class and replace
+``'django.contrib.staticfiles'`` with that class path in your
+:setting:`INSTALLED_APPS` setting:
 
 .. code-block:: python

[Tobias Kunze]

I'd say so, thanks! (This issue was actually mentioned via #django on freenode by jleclanche.)

[Tim Graham <timograham@…>]

In ec4a6b33:

Fixed #26946 -- Clarified instructions for customizing collectstatic's ignore_patterns.

[Tim Graham <timograham@…>]

In 13c107b:

[1.10.x] Fixed #26946 -- Clarified instructions for customizing collectstatic's ignore_patterns.

Backport of ec4a6b33a9af7f7c266c92b4ffb9b0a49ee5b1eb from master