Improve documentation around class naming

[ksours]

Sometime between phpunit8.5.2 and phpunit9.4.2 a requirement was added that the class name match the filename. This is not listed on the "backwards incompatible changes" here: https://phpunit.de/announcements/phpunit-9.html (and is not clear from the class name issues mentioned).

Similarly, if it's mentioned in the documentation I couldn't find it. I had to inspect the code to figure out that the requirements actually where (and would not have guessed that the actual requirement is that the filename be a prefix of the classname).