Augment documentation.

The documentation of MultithreadInfo was unintelligible. It was also
incomplete in some places.
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.