Gio::OutputStream: write_async(), write_all_async(): Improve docs.
Gio::OutputStream: write_async(), write_all_async(): Improve docs.

Add the warning about the buffer needing to stay valid because it
is not copied, adding to the C API here:
https://git.gnome.org/browse/glib/commit/gio/goutputstream.c?id=0f800cd1a863bc0c3e51c03592e2fb1ffbda8982

johne53/MB3Glibmm,GNOME/glibmm,GNOME/glibmm,johne53/MB3Glibmm,GNOME/glibmm,GNOME/glibmm,johne53/MB3Glibmm,GNOME/glibmm,johne53/MB3Glibmm