* Add proper XML header.
    Note: this is something which only recently became an option as there previously was no schema available.
* Style: have `CDATA` tags on their own lines.
    This makes the XML code style consistent with currently existing XML docs.
* Expand the issue description.
* Add descriptions for the code samples.
* Fix code sample for what the sniff is looking for.
    Make it clearer that a double underscore prefix is allowed for magic methods.
* Add `<em>` tags to the code samples to allow for highlighting the specific issue the sniff is looking for.
* Move the `link` element to be an attribute of the description element to comply with the XML schema.