* 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 and add a second description section about the "manual inspection" warning.
* Use `Cross-version compatible` and `Cross-version INcompatible:` 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.
* Fix code sample for what the sniff is looking for and add a lot more code samples.
    Make it clearer which type of code is affected by this change in PHP and when the issue doesn't apply.
* Improve the code sample descriptions.
* 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.