Clarify Code Coverage in CI Examples

[jrchamp]

I think that none is the default int the dist files (both GHA and Travis) because those dist files don't use any phpunit coverage option.

Only if some phpunit coverage option is used is when the pcov => xdebug => phpdbg fallback happens and they are used.

Surely we could:

• Make the GHA dist file clearer (I think the Travis one is better).
• At some point, go deprecating the phpdbg option. Maybe warning... that is a can of worms, specially with PHP 8.x versions.

Originally posted by @stronk7 in #204 (comment)