spec: Clarify behaviour of o.f.D.P.GetAll

spec: Clarify behaviour of o.f.D.P.GetAll

Clarify its intended behaviour in two situations:
 • For interfaces which have no properties.
 • Where some properties are not visible to the caller (due to access
   control, for example).

The intention here is for this behaviour to be mandatory, but given that
this is quite late on in the specification’s life, and various D-Bus
libraries like dbus-glib and telepathy-glib cannot support access
control at a per-property level, for example. GDBus can, although it’s
questionable whether this is a good idea. Deliberately leave the
specification open to allow access control at a higher level as well
(such as per-(object, interface)).

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=36190
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>