Add note about Concatenation spacing

As per brians request in https://github.com/joomla/joomla-cms/pull/4172
what do you think about:

```php
$hello = 'Hello ';
$world = 'World';
$helloworld = $hello . $world;
echo $helloworld; => "Hello World"
```

or a other example:
```php
JPATH_ROOT . 'foo/foo.php';
```

a third one:
```php
$id = 1;
echo JRoute('index.php?option=com_foo&task=foo.edit&id=' . (int) $id);
```