Add description and usage note about decorators in URLconf to the url dispatcher docs

[Ramiro Morales]

At least one of the three persons reporting in #5920 used the cache_page decorator in the URLconf but passed the view parameter as a string and not as a callable object itself.
The patch a) copies the usage case description from the 0.96 release notes to the main documentation and b) adds a note about that specific gotcha.

[James Bennett]

I'm not sure we really should document this; the fact that decorators require a callable as an argument is pretty basic Python, and it's not really Django's job to serve as a Python tutorial.