docs: Clarify that login1 signals are not emitted for convenience objects

docs: Clarify that login1 signals are not emitted for convenience objects

While this is obvious if you spend a few minutes thinking about how
D-Bus signals work (in this case, they are broadcast from a system
service, so cannot apply to a specific user/session/seat), it’s a bit
easy to overlook this while putting code together which uses the login1
D-Bus API, so it’s helpful to point this hazard out specifically in the
docs.

The signals can only be emitted on the canonical objects. The
convenience objects are useful for method calls, as the calling context
can be used to dereference ‘self’ and ‘auto’, but this can’t work for
signals.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>