Use "var" instead of "void" as return type when no api documentation is available

This pull request changes the return type for methods without apidoc from `Type::$VOID` (think: *does not return anything*) to `Type::$VAR` (think: *may return anything*).

```php
<?php
  class Test extends Object {
    public function fixture() { }
  }
?>
```

### Old behaviour:
```sh
$ xp -w 'XPClass::forName("Test")->getMethod("fixture")->getReturnType();'
lang.Type<void>
```

### New behaviour:
```sh
$ xp -w 'XPClass::forName("Test")->getMethod("fixture")->getReturnType();'
lang.Type<var>
```

### BC
The behaviour for this is unchanged:
```php
<?php
  class Test extends Object {

    /**
     * The fixture
     */
    public function fixture() { }
  }
?>
```

As the is no `@return` tag, the return type is `void`.

### Motivation
This is useful during RAD; especially for REST services, where the `void` return type will lead to a `HTTP/1.1 204 No Content`.