Some tweaks to the staticfiles docs to clarify things for new users. Thanks Jannis and brutasse for review and discussion.

* Rearranged 'in a nutshell' usage docs to clarify app-dirs vs STATICFILES_DIRS, initially focus on the simplest path to working local dev, and remove the need for repetitive 'you don't need this in local dev' notes.
* Added docs on using staticfiles serve view for non-staticfiles (e.g. user-uploaded files).

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15380 bcc190cf-cafb-0310-a4f2-bffc1f526a37