cameo: documented X/Y translation commands, some small improvements

- README: described the X/Y translation commands
- lang.y: print an error if the lower bound if larger than the upper bount
  in an "align" command
- lang.y: mention that "array" doesn't work as expected if in imperial mode

cameo/lang.y

@@ -139,6 +139,11 @@ command:
 			int ref = $2;
 			double x, y;
 
+			if ($3 > $5)
+				yyerror("left edge > right edge");
+			if ($4 > $6)
+				yyerror("bottom > top");
+
 			ref_pick_xy(ref, $3, $4, $5, $6, &x, &y);
 			align(ref, x, y);
 		}
@@ -147,6 +152,7 @@ command:
 			double x = $2*$4;
 			double y = $3*$5;
 
+			/* @@@ known bug: doesn't work if not in metric mode */
 			translate(x, y, 0);
 		}
 	| TOK_CLEAR