doc: doc imitating the old behavior of http.Server.keepAliveTimeout

Documenting the best way to imitate the old behavior saves time for people
migrating from older versions. (E.g. for unexpected ECONNRESET)

It isn't immediately obvious if earlier nodejs versions behaved the same
way as nodejs 8 does with keepAliveTimeout = 0.
From 0aa7ef59, it seems like they behave
the same way.

Related to issues such as #13391 that show up when migrating to node 8

PR-URL: https://github.com/nodejs/node/pull/17660


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>