gdatainputstream: Document the returned string is always nul-terminated
gdatainputstream: Document the returned string is always nul-terminated

Signed-off-by: Philip Withnall <withnall@endlessm.com>
Reviewed-by: nobody

https://bugzilla.gnome.org/show_bug.cgi?id=742124

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib