net/http: document CanonicalHeaderKey from Header

And remove some unnecessary textproto references. (The net/http
package's CanonicalHeaderKey just calls textproto's
CanonicalMIMEHeaderKey)

Fixes #28894

Change-Id: Ibd277893a168368c593147a2677ad6130870cb88
Reviewed-on: https://go-review.googlesource.com/c/152157


Reviewed-by: Ian Lance Taylor <iant@golang.org>