Fix the docs for gtk_recent_info_get_application_info()

Fix the docs for gtk_recent_info_get_application_info()

The documentation for the function says that the app_exec string
should be freed, but we return a pointer to the internal string
without duplicating it. Since the app_exec string is valid as long
as the GtkRecentInfo is valid the documentation should be fixed
and the out argument should be constified. Fixes #584832.