Improve the wording of an exception.

I ran into this exception and couldn't figure out what's wrong without looking at the source code.
Improve the documentation of VectorTools::create_point_source_vector().

The previous version of the documentation was not exactly clear as to what the 'orientation' argument actually referred to.
Update documentation a bit.

Reading through the documentation of some of the functions in DoFTools, there are a number of functions for which I can't quite figure out what exactly they do. This updates a couple of places, but more work remains. In particular, I have no idea from the description of what DoFTools::make_single_patch() does.
Merge pull request #36 from bangerth/doc-fixes-4

Update documentation a bit.
Merge pull request #33 from bangerth/doc-fixes

Improve the wording of an exception.
Merge pull request #35 from bangerth/doc-fixes-3

Improve the documentation of VectorTools::create_point_source_vector().