Update the documentation of VectorTools::project_boundary_values().

This had apparently confused a user on the mailing list earlier today, so structure the
documentation better.
Merge pull request #34 from bangerth/doc-fixes-2

Update the documentation of VectorTools::project_boundary_values() to make it clearer.
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().
Add more functions that deal with the infrastructure for solving local problems on patches around cells.

The functions DoFTools::count_dofs_on_patch() and DoFTools::map_global_dofs_to_patch_indices() help set up linear systems on patches. This is joint work with Arezou Ghesmati.
Merge pull request #37 from bangerth/add-patch-generators

Infrastructure functions for local problems on patches

The functions DoFTools::count_dofs_on_patch() and
DoFTools::map_global_dofs_to_patch_indices() help set up
linear systems on patches.
This is joint work with Arezou Ghesmati.
Fix numerous doxygen markup problems.

Doxygen got bent all out of shape in various places because formulas were opened
but not closed, and a number of other markup problems as well.