Clarify cleaner doc on setting comments=False

[JocelynDelalande]

Maybe the actual way to go is to remove this rule you seem not to be sure about. The sure thing is that the effect is quite unexpected for the library user: "I cannot disable comments removing".

[scoder]

I agree that it's unexpected. Not sure if we should break code by fixing this (it's been like that forever), but it feels like we should fix it in the code, rather than documenting weird behaviour.

[mlissner]

It's worth reviewing this too: https://bugs.launchpad.net/lxml/+bug/1882606

Cleaner.clean_html() uses the default parser internally, which strips comments and PIs – before even passing the result through the cleaner. I admit that that may appear counter-intuitive. PR welcome to configure the parser based on the relevant Cleaner options.