Add hint to Diff.__iter__()

[rmoehn]

I'm not the guy who looks at examples in the first place and I guess
there are other people like me. When I wanted find out how to get
information out of a Diff, I looked at the documented methods and didn't
find anything. Only later @cmn showed me the [p for p in diff] example
in the documentation. Add a short piece of information that gives a hint
to those who prefer the dry API docs.

[rmoehn]

Sorry for using the wrong username. 😬