add exit handler

doc/riverguile.1

@@ -78,6 +78,9 @@ of the system being idle is over.
 Multiple idle handlers can be installed.
 Note: All idle events relate to the first advetised seat.
 As of now, river only supports a single seat anyway.
+.P
+The key \fBexit\fR installs a handler which is called when riverguile exits.
+The procedure takes no arguments.
 .
 .
 .SH EXAMPLE