* 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 a little.
* Use `Cross-version compatible` and `PHP </> #.#` style prefixes in the code samples.
    This improves consistency with the other XML docs and makes the code samples more descriptive in the context of PHPCompatibility.
* Add `<em>` tags to the code samples to allow for highlighting the specific issue the sniff is looking for.