doc, http2: add sections for server.close()

Clarify current behavior of http2server.close() and
http2secureServer.close() w.r.t. perceived differences
when compared with httpServer.close().

Fixes: https://github.com/nodejs/node/issues/19711

PR-URL: https://github.com/nodejs/node/pull/19802


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>