… tweaks

* 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.
* 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.
* Fix code sample for what the sniff is looking for.
    Make it clearer that a PHP 4 style constructor method accompanied by a `__construct()` method will not be flagged.
* Add `<em>` tags to the code samples to allow for highlighting the specific issue the sniff is looking for.
* Remove code samples related to things PHPCompat doesn't check/sniff for.
    While the code samples about calling direct/indirect parent constructors is good and valid information, the sniff does not look for that and PHPCompatibility is incapable of flagging this as it would involve, in most cases, cross-file analysis.
    The documentation is only supposed to inform the users about what the PHPCompatibility sniff flags.
* Move the `link` element to be an attribute of the description element to comply with the XML schema.