Clarify in documentation that runserver is overridden for django.contrib.staticfiles

[David Beitey]

In the main ​static files documentation, the serving process is generally explained that runserver will automatically serve files if DEBUG is set to true. This isn't quite the full story because it's not the _original_ runserver that is being used, and rather it's the one from django.contrib.staticfiles. This gets explained if you ​dig deeper, but it isn't clear what's happening before getting this deep.

PR at ​https://github.com/django/django/pull/11121

This PR makes a clarification to the wording of this section in the docs to explain how runserver is working its magic, and that if you have a custom runserver command (like I did), you need a special strategy.

[Carlton Gibson]

Closing ​as per comment on PR. (Roughly: too much of an edge-case to be worth the complication, and advising the use of collectstatic in development not the best way to go.)