man: document unprivileged is not for reading properties

man: document unprivileged is not for reading properties

Document the fact that read-only properties may not have the flag
SD_BUS_VTABLE_UNPRIVILEGED as that is not obvious especially given the
flag is accepted for writable properties.

Based on the check in `add_object_vtable_internal` called by
`sd_bus_add_object_vtable` (as of the current tip of the main branch
f7f5ba019206cacd486b0892fec76f70f525e04d):

    case _SD_BUS_VTABLE_PROPERTY: {
            [...]
            if ([...] ||
                [...]
                (v->flags & SD_BUS_VTABLE_UNPRIVILEGED && v->type == _SD_BUS_VTABLE_PROPERTY)) {
                    r = -EINVAL;
                    goto fail;
            }

(where `_SD_BUS_VTABLE_PROPERTY` means read-only property whereas
`_SD_BUS_VTABLE_WRITABLE_PROPERTY` maps to writable property).

This was implemented in the commit
adacb9575a09981fcf11279f2f661e3fc21e58ff ("bus: introduce "trusted" bus
concept and encode access control in object vtables") where
`SD_BUS_VTABLE_UNPRIVILEGED` was introduced:

    Writable properties are also subject to SD_BUS_VTABLE_UNPRIVILEGED
    and SD_BUS_VTABLE_CAPABILITY() for controlling write access to them.
    Note however that read access is unrestricted, as PropertiesChanged
    messages might send out the values anyway as an unrestricted
    broadcast.

(cherry picked from commit 3ca09aa4dd57327989eceb1298754601046ac041)
(cherry picked from commit cd727031a4daafe19f491df360c512433562f469)