Clarify response header behavior in the case of a redirect

[kgriffs]

Clarify the docs re this comment in chat from a community member:

Hi everyone - I have a documentation question - https://falcon.readthedocs.io/en/2.0.0/api/redirects.html What does "headers" parameters do in these exceptions? Merge with original request headers? Replace them?

A.

the headers that are merged are the ones set in the response, not the original request ones https://github.com/falconry/falcon/blob/871d6bbe5b76d63fddae566b2682b321603af06b/falcon/app.py#L930-L953