Add note about Concatenation spacing

As per brians request in https://github.com/joomla/joomla-cms/pull/4172
Change this sentence and I think we're good.

There should always be a space before and after the concatenation operator ('.').  For example:

`blah`

Maybe we should also add an example for when the concat is the first or last character on a line?

`If the concatenation operator is the first or last character on a line, both spaces are not required.  For example:`

```php
$id = 1
echo JRoute::_(
    'index.php?option=com_foo&task=foo.edit&id=' . (int) $id
    . '&layout=special'
);
```