spec: Mention the well-known fallback path for the session bus
Multiple implementations (including dbus
itself and zbus
use the
path $XDG_RUNTIME_DIR/bus
when $DBUS_SESSION_BUS_ADDRESS
is unset.
This fallback path is quite convenient, but not documented as part of the specification. This changeset adds a mention of this de-fact standard fallback.
Fixes: #449