net/http: document that after Hijack, Request.Body is invalid

net/http: document that after Hijack, Request.Body is invalid

We can make it panic with a more explicit and readable error message
during Go 1.10, but document it for now. This has always been the
case; it's not a new rule.

Updates #20933

Change-Id: I53c1fefb47a8f4aae0bb32fa742afa3a2ed20e8a
Reviewed-on: https://go-review.googlesource.com/50634
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>