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
f7f5ba0):

    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
adacb95 ("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 3ca09aa)
(cherry picked from commit cd72703)
(cherry picked from commit f694a84)
(cherry picked from commit 0aea62b)
(cherry picked from commit 413d82c)
(cherry picked from commit a85962c)

Author: Katariina Lounento

man/sd_bus_add_object.xml

@@ -593,6 +593,9 @@
           <constant>org.freedesktop.systemd1.Privileged</constant> annotation with value
           <literal>true</literal> will be shown in introspection data.</para>
 
+          <para><constant>SD_BUS_VTABLE_UNPRIVILEGED</constant> may not be applied to read-only properties,
+          but read access (to both read-only and writable properties) is always unrestricted.</para>
+
           <para>Note that this page describes checks implemented in the D-Bus client. The D-Bus server has an
           additional policy that may permit or deny connections, see
           "CONFIGURATION FILE" in