doc: improve http.setHeader and getHeader typeinfo

http.setHeader() coerces input values.
http.getHeader() returns the type as passed to setHeader().

PR-URL: https://github.com/nodejs/node/pull/19902
Fixes: https://github.com/nodejs/node/issues/13825


Reviewed-By: Chen Gang <gangc.cxy@foxmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>