doc: value choice for imitating the old behavior of http.Server.keepAliveTimeout

Checklist

• documentation is changed or added
• commit message follows commit guidelines

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 (closed) that show up when migrating to node 8

Affected core subsystem(s)

doc