Update documentation of this file.

- Rename a documentation section to be more descriptive.
- Reshuffle some more functions and group them into appropriate categories. Not all functions are in categories yet, though, see http://www.dealii.org/developer/doxygen/deal.II/namespaceDoFTools.html
Merge pull request #9 from bangerth/doc-fixes

Update documentation in include/deal.II/dofs/dof_tools.h

- Rename a documentation section to be more descriptive.
- Reshuffle some more functions and group them into appropriate categories. Not all functions are in categories yet, though, see http://www.dealii.org/developer/doxygen/deal.II/namespaceDoFTools.html