describe default behavior of scroll

scroll.1

@@ -30,6 +30,11 @@ The
 utility saves output lines from the child
 .Ar program
 to use them for scrollback.
+If
+.Ar program
+is not set,
+.Nm
+starts the users default shell.
 .Pp
 The options are as follows:
 .Bl -tag -width Ds