Add documentation comments

java4/frontend/PrettyPrintUtil.jrag

@@ -266,6 +266,7 @@ aspect PrettyPrintUtil {
 
   syn boolean AbstractDot.needsDot() = !(getRight() instanceof ArrayAccess);
 
+  /** The operator string used for pretty printing this expression. */
   syn String Binary.printOp();
   eq MulExpr.printOp() = "*";
   eq DivExpr.printOp() = "/";
@@ -287,6 +288,7 @@ aspect PrettyPrintUtil {
   eq EQExpr.printOp() = "==";
   eq NEExpr.printOp() = "!=";
 
+  /** The operator string used for pretty printing this expression. */
   syn String AssignExpr.printOp();
   eq AssignSimpleExpr.printOp() = "=";
   eq AssignMulExpr.printOp() = "*=";