cameo: documented "area"; miscellaneous cleanup

Also removed the tool diameter argument. We use the diameter from the
path, like "offset" does. In the long run, this convention probably
doesn't make sense, though.

cameo/test/doit

@@ -1,6 +1,6 @@
 #!/bin/sh
 ../cameo <<EOF
 gnuplot 3 in.gp
-area 3 0
+area 0.13
 write out.gp
 EOF