Rewrite the documentation of DoFTools::compute_intergrid_constraints().

The previous version of the documentation basically stated why the function exists, not what it does. This is the focus of the new version. I also indented the arguments of this function and the (related) following one.
Merge pull request #13 from bangerth/doc-fixes

The previous version of the documentation basically stated why the function exists, not what it does. This is the focus of the new version. I also indented the arguments of this function and the (related) following one.