Document the support for replacing placeholders in @testdox text with data provider values

[sebastianbergmann]

It would be good to document @ https://phpunit.readthedocs.io/en/9.5/annotations.html?highlight=testdox#testdox that one can also use $_dataName in the @testDox line, and it's the value of the key from the data provider's array, ie:

/**
@testDox $_dataName ($testCase)
@dataProvider provideCases
*/
function testSomething($testValue) {
    // etc
}

function provideCases() {
    return [
        'testing an OK value' => ['testValue' => 42],
        'testing a bad value' => ['testValue' => -1]
    ];
}

Yields this sort of thing in the test output:

* testing an OK value (42)
* testing a bad value (-1)

It would also be good to document the @testDox usage at https://phpunit.readthedocs.io/en/9.3/writing-tests-for-phpunit.html#data-providers, given it's reasonably closely related to some of the other documented functionality there.

(Mostly putting this here in case someone lands here from a search, wondering how to leverage that key name in the @testDox line.

Originally posted by @adamcameron in sebastianbergmann/phpunit#3196 (comment)