java: beware not to alias the locations of the various symbols

* examples/java/calc/Calc.y, tests/calc.at, tests/local.at
(getStartPos, getEndPos): Always return a new object.
* doc/bison.texi: Clarify this.

tests/calc.at

@@ -352,11 +352,11 @@ m4_define([AT_CALC_YYLEX(java)],
   Position end = new Position (1, 0);
 
   public Position getStartPos () {
-    return start;
+    return new Position (start);
   }
 
   public Position getEndPos () {
-    return end;
+    return new Position (end);
   }
 
 ]])[