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

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http://techytim.com/django/17551/topics/http/urls.html#url-namespaces
Wouldn't it be preferable to recommend explicit argument names? I'd even consider deprecating both positional and tuple syntaxes, and enforcing keyword-only for the namespace arguments. `f(a,b,c)` being equivalent to `f((a,c,b))` strikes me as a footgun (even more so when b=c is a useful and common pattern).