Clarify documentation and verify what I say by adding tests.
Merge pull request #156 from bangerth/clarify-docs

Clarify documentation and verify what I say by adding tests. This documents has_active_dofs() in parallel.
Move declarations in DoFHandler from protected to private.

DoFHandler still had some of its member variables and functions in a protected
section from back when we still had MGDoFHandler derive from it in a meaningful
way. This is now no longer necessary, so remove the protected section and instead
sort things in some sort of natural way into the private section.
Merge pull request #161 from bangerth/private-protected-cleanup

Move declarations in DoFHandler from protected to private.
DoFHandler still had some of its member variables and functions in a protected
section from back when we still had MGDoFHandler derive from it in a meaningful
way. This is now no longer necessary, so remove the protected section and instead
sort things in some sort of natural way into the private section.
Remove unnecessary #include.
Merge pull request #162 from bangerth/remove-unnecessary-include

Remove unnecessary include
Update documentation in a couple of places.
Remove uses of <tt>...</tt> in a number of places in favor of more
appropriate markups.
Reindent all short comments to standard length.