OptionContext: Add API documentation based on the C docs.

* glib/src/optioncontext.hg: Add class overview documentation
  based on the C documentation, but reworded to match our C++ API.
  This should now provide some clue about where to start other
  than with our (well hidden) example code.
* glib/src/optiongroup.hg: Add documentation for the add_entry*()
  methods.