services.dbus module: more complete docstring

The docstring for the `services.dbus.packages` configuration option only
mentioned one directory, but the implementation actually looked for DBus
config files in four separate places within the target packages. This
commit updates the docstring to reflect the actual implementation
behaviour.

nixos/modules/services/system/dbus.nix

···
         default = [ ];
         description = ''
           Packages whose D-Bus configuration files should be included in
-          the configuration of the D-Bus system-wide message bus.
-          Specifically, every file in
+          the configuration of the D-Bus system-wide or session-wide
+          message bus.  Specifically, files in the following directories
+          will be included into their respective DBus configuration paths:
           <filename><replaceable>pkg</replaceable>/etc/dbus-1/system.d</filename>
-          is included.
+          <filename><replaceable>pkg</replaceable>/share/dbus-1/system-services</filename>
+          <filename><replaceable>pkg</replaceable>/etc/dbus-1/session.d</filename>
+          <filename><replaceable>pkg</replaceable>/share/dbus-1/services</filename>
         '';
       };