g_file_new_for_commandline_arg: clarify encoding

Add a note to the documentation for g_file_new_for_commandline_arg()
that this function is intended to operate on strings already in the GLib
filename encoding on Windows.

This has been the case for a long time, but this documents the
requirement.

https://bugzilla.gnome.org/show_bug.cgi?id=722025