[Bug 797399] Fix wiki.models.Document.from_url()

https://bugzilla.mozilla.org/show_bug.cgi?id=797399

There is a misleading sentence in the docstring of `Document.from_url()`:

> However, if you pass host_safe=True, we assume the URL's host is the one serving
> Documents, which comes in handy for analytics whose metrics return host-having URLs.

I can't find the `host_safe` parameter in the wiki application.