add documentation

manual/actmodlinearmovement.tex

@@ -40,3 +40,4 @@
 /scene/obj/t0 (d)
 /scene/obj/vpt (ddddddd)
 \end{verbatim}
+Note that only setting the last OSC variable \verb!/scene/obj/vpt! ensures an atomic operation of setting the variables. If you set it variable by variable, you may get undefined (and possibly extreme) intermediate values.