Remove #includes of multithread_info.h in files that don't need it.
Merge pull request #5 from bangerth/doc-fixes

Augment documentation.

The documentation of MultithreadInfo was unintelligible. It was also
incomplete in some places.
Merge pull request #6 from bangerth/remove-multithread_info.h-includes

Remove #includes of multithread_info.h in files that don't need it.
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