Gio::DBus::AuthObserver: Add example from the C API docs in class docs

* gio/src/dbusauthobserver.hg: Add the example from the C API docs,
translated to C++.
Gio::Mount::guess_content_type_finish(): Correct the documentation

* gio/src/mount.hg: guess_content_type_finish(): Adapt the docs to C++
instead of copying directly from glib.
Gio::ThemedIcon::get_names(): Use _WRAP_METHOD

* gio/src/themedicon.[hg|ccg]: Use _WRAP_METHOD instead of hand-written
code. gmmproc does not complain.
Glib::ustring: Delete an obsolete TODO comment

* glib/glibmm/ustring.cc: The comment added 2007-08-12 by commit
https://git.gnome.org/browse/glibmm/commit/?id=132b0ceb8c30ef82a6c11635eff8e7b41feba357
is obsolete or a misunderstanding. A std::wostream requires a wchar_t string
as input from the C++ code, but the character encoding of a written file is
determined by the std::wostream's std::locale.