Document that clients can expect to receive notifications about some
attributes which they have not subscribed to, if it is more efficient
for the server to emit those alongside attributes the client //has//
subscribed to.

Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Signed-off-by: Philip Withnall <philip.withnall@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D3500