net/http: clarify that AddCookie only sanitizes the Cookie being added

net/http: clarify that AddCookie only sanitizes the Cookie being added

AddCookie properly encodes a cookie and appends it to the Cookie header
field but does not modify or sanitize what the Cookie header field
contains already. If a user manualy sets the Cookie header field to
something not conforming to RFC 6265 then a cookie added via AddCookie
might not be retrievable.

Fixes #38437

Change-Id: I232b64ac489b39bb962fe4f7dbdc2ae44fcc0514
Reviewed-on: https://go-review.googlesource.com/c/go/+/235141
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>