net/http: fix and normalize the [Server.][ListenAnd]Serve[TLS] docs

The only inaccurate part was the HTTP/2 caveat in Server.ServeTLS, which
only applies to the plain Serve variant.

The restriction implemented in shouldConfigureHTTP2ForServe is not on
the setupHTTP2_ServeTLS codepath because ServeTLS owns the tls.Listener,
so we fix it for the user instead of disabling HTTP/2.

Fixes #24607

Change-Id: Ie5f207d0201f09db27bf81b75535e5f6fdaf91e2
Reviewed-on: https://go-review.googlesource.com/103815


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>