Fixed #17551, #20734 -- Added examples to URL namespacing docs.

If you prefer reading rendered HTML, all edits are from this section to the bottom of the page.
http://techytim.com/django/17551/topics/http/urls.html#url-namespaces
This could be a little complex in that things like `AdminSite.urls` would have to return a dict (unless I'm missing another solution) which leads to `include(**admin.site.urls)`. Maybe it would be better (although backwards incompatible) to reorder the params in `include()` so they match the tuple syntax. It may be less invasive to continue allowing tuples.