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?).