Add some clarification for some files under config/*

Add some clarification for some files under config/*

I didn't add a lot of them because I am not very familiar about how they work and I am not sure if we really need to explain files like config/database.yml because if you know Rails you must know how that works.

At any rate, I think this is a beginning and we could keep documenting more files in the future.

Closes #18373 (closed)