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.

examples/java/calc/Calc.y

@@ -99,11 +99,11 @@ class CalcLexer implements Calc.Lexer {
   Position end = new Position (1, 0);
 
   public Position getStartPos () {
-    return start;
+    return new Position (start);
   }
 
   public Position getEndPos () {
-    return end;
+    return new Position (end);
   }
 
   public void yyreportSyntaxError (Calc.Context ctx)
@@ -175,6 +175,12 @@ class Position {
     column = t;
   }
 
+  public Position (Position p)
+  {
+    line = p.line;
+    column = p.column;
+  }
+
   public void set (Position p)
   {
     line = p.line;