Better document (just response?) headers

While we know the effect of `Header` (and its general variant that allows modifiers) when used for request headers, we don't have much doc or examples when using `Headers '[ ... ]` in response headers. Whether the response headers necessarily have to be there or not (no, see `noHeader`), whether modifiers affect the necessity for headers to be there or not, from server & client perspective, etc.

It would be great to have tests that cover all the cases/angles and a slightly more helpful documentation around this whole topic (in the haddocks for sure and perhaps the tutorial/cookbook?).