net/http: document Response.Header values that are subordinate to other fields

I noticed that Content-Length may appear in http.Response.Header, but the docs
say it should be omitted.  Per discussion with bradfitz@, updating the docs to
indicate that the struct fields are authoritative.

Change-Id: Id1807ff9d4ba5de425d8b147205f29b18351230f
Reviewed-on: https://go-review.googlesource.com/36842


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