update documentation for new API

doc/riverguile.1

@@ -50,6 +50,7 @@ the x and y coordinates of the window as well as its width and height.
 Window positions and dimensions get applied to rivers window list top to bottom.
 Note that the numerical values do not need to be exact, riverguile takes care
 of rounding and casting for you.
+A layout demand handler must be installed, otherwise riverguile will exit.
 .P
 The key \fBuser-command\fR install a handler for user commands, which are the
 strings a user can send to layout generators.