Enhance code comments

[atsju]

Who has never read his own code and didn't understand it a year after ?

This repo becomes bigger, greater and more used every day and even if I'm able to write some cmake code I do not consider myself as expert. It is often quite difficult for me to understand exactly what the code does.

I think a minimum of comment should be added and every new PR should have (in an ideal world) :

• comments for the added code
• documentation update if behavior changes

Also the original code should be commented in the futur.

Comment shall not explain what set does on every line but regex, general flow and "advanced" cmake features (I know this means nothing depending on people) could definitely be.

(#190 mentions this problem).

[gudnimg]

I agree. Its hard to see what the code is doing sometimes 😅