doc: running on different seat with libseat

When using the libseat launcher, there is one more detail to take care:
stop libseat from managing the VT.

A normal user does not have permissions to manage a VT, so launching
would just fail. In this use case you also do not want to be managing
the VT, because your normal desktop is already owning the seat
associated with the VT.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

doc/sphinx/toc/running-weston.rst

@@ -174,7 +174,14 @@ Then, weston can be run by selecting the DRM-backend and the seat ``seat-insecur
 
 ::
 
-        ./weston -Bdrm-backend.so --seat=seat-insecure
+        SEATD_VTBOUND=0 ./weston -Bdrm-backend.so --seat=seat-insecure
+
+This assumes you are using the libseat launcher of Weston with the "builtin"
+backend of libseat. Libseat automatically falls back to the builtin backend if
+``seatd`` is not running and a ``logind`` service is not running or refuses.
+You can also force it with ``LIBSEAT_BACKEND=builtin`` if needed.
+``SEATD_VTBOUND=0`` tells libseat that there is no VT associated with the
+chosen seat.
 
 If everything went well you should see weston be up-and-running on an output
 connected to that DRM device.