PDO: improve PDO::ATTR_STRINGIFY_FETCHES description

[pereorga]

1. Standardizes definition in both constants and setattribute pages. I'm not sure this duplication is ideal, but that is how the PDO documentation is currently structured.
2. Update text to reflect that it's not the numeric values that get converted, it's all non-null values. (E.g. boolean values get converted too.). I imagine the behavior could theoretically be different depending on the PDO driver, and that the behavior may have been different in the past too, but that seems to be the case nowadays in all core drivers, and so I think it's useful to state it.
3. Specify the default. Again, this may be driver-specific (an extension may override it), but at least that seems to be the case in all existing supported drivers.
4. Mention ATTR_ORACLE_NULLS. As a developer, it can be useful to enforce all types to be string, and that could be wrongly assumed to be done by this constant (named ATTR_STRINGIFY_FETCHES).

[pereorga]

I noticed that https://www.php.net/manual/en/pdo.constants.php lists all boolean constants as int. I guess there may be an historical reason for this, happy to update it in another PR if it makes sense.

[kamil-tekiela]

I noticed that https://www.php.net/manual/en/pdo.constants.php lists all boolean constants as int. I guess there may be an historical reason for this, happy to update it in another PR if it makes sense.

Which one of them is not null?

[kamil-tekiela]

E.g. boolean values get converted too.

That is only true as of PHP 8.1. I don't know if it's worth mentioning in the documentation though since only PGSQL has boolean type.

[kamil-tekiela]

LGTM!

[pereorga]

I noticed that https://www.php.net/manual/en/pdo.constants.php lists all boolean constants as int. I guess there may be an historical reason for this, happy to update it in another PR if it makes sense.

Which one of them is not null?

Never mind, I got confused with the type of the constants