Document new PDO classes and methods

[Girgias]

Reference: https://wiki.php.net/rfc/pdo_driver_specific_subclasses and https://wiki.php.net/rfc/pdo_driver_specific_parsers

Still needs MySQLi, PGSQL, and OCI8 (although that one is going to be low priority)

@SakiTakamachi can you document, either as inline comments or in a follow-up PR the transaction attributes?

[Girgias]

@sim1984 you are probably the best person to review the FireBird changes, I took some of the wording for the attributes (based on Saki's initial text) from https://ib-aid.com/articles/php-8-4-improvements-when-working-with-modern-firebird-versions

[Girgias]

@KentarouTakeda could you have a look at the Pdo\Pgsql docs?

[KentarouTakeda]

@Girgias
I tried to write as best as I could.

Please note that my use of English and xml syntax may not be very good in some places.

[Girgias]

@kamil-tekiela can you review the Pdo\Mysql docs? (I don't know what the dedicated method does as it is documented no where...)

[kamil-tekiela]

Partial review