Add a note to the documentation of
`g_dbus_connection_signal_unsubscribe()`, `g_bus_unwatch_name()` and
`g_bus_unown_name()` warning about the need to continue iterating the
caller’s thread-default `GMainContext` until the
unsubscribe/unwatch/unown operation is complete.

See the previous few commits and #1515

 for an idea of the insidious bugs
that can be caused by not iterating the `GMainContext` until
everything’s synchronised.

Signed-off-by: Philip Withnall <withnall@endlessm.com>