[PHPUnit bridge] Add examples for SYMFONY_PHPUNIT_REQUIRE

[fattybenji]

The SYMFONY_PHPUNIT_REQUIRE environment variable allows us to specify additional packages to install. Documentation for this variable has been added in this commit but no examples have been given.

We can find an example in the original PR for the feature, but there seems to be no mention of the possibility (or not) to install multiple packages. Here is the example value given in the PR: phpspec/prophecy-phpunit:^1.0.

Would it be possible to add one or two examples to the documentation ?

[javiereguiluz]

Closing as fixed in #17968.