net/http: update Serve docs on when HTTP/2 is enabled
net/http: update Serve docs on when HTTP/2 is enabled

Contains portions and modified portions of CL 103815

Fixes #24607

Change-Id: Ic330850a0f098f183315f04ea4780eded46c5b77
Reviewed-on: https://go-review.googlesource.com/125515
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/go,CAFxX/go,golang/go,cloudflare/go,josharian/go,golang/go,CAFxX/go,nawawi/go,CAFxX/go,josharian/go,CAFxX/go,CAFxX/go,nawawi/go,josharian/go,akutz/go,cloudflare/go,golang/go,akutz/go,akutz/go,nawawi/go,nawawi/go,josharian/go,cloudflare/go,akutz/go,josharian/go,josharian/go,CAFxX/go,akutz/go,CAFxX/go,cloudflare/go,golang/go,golang/go,nawawi/go,golang/go,nawawi/go,cloudflare/go,akutz/go,cloudflare/go,CAFxX/go,cloudflare/go,akutz/go,josharian/go,josharian/go,golang/go,akutz/go,nawawi/go,cloudflare/go,nawawi/go