'Unknown section' warning should say where it is

[takluyver]

One thing I frequently have to fix is people using incorrect section headers, e.g. 'Return' instead of 'Returns'. The warning in this case just looks like this:

Unknown section Return

Most Sphinx warnings do a reasonable job of telling me which file or docstring the problem was in, which makes it much easier to fix.

[marscher]

this applies to all sections. There is no useful information given, where to find this. Of course one can grep the location, but why now directly providing it?

[jnothman]

Fixed in #101