doc: describe current HTTP header size limit

Document that the limit was changed from 80KB to 8KB in 18603524.

Fixes: https://github.com/nodejs/node/issues/24693

PR-URL: https://github.com/nodejs/node/pull/24700


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>