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/36842Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/response.go

@@ -37,9 +37,10 @@ type Response struct {
 	// Header maps header keys to values. If the response had multiple
 	// headers with the same key, they may be concatenated, with comma
 	// delimiters.  (Section 4.2 of RFC 2616 requires that multiple headers
-	// be semantically equivalent to a comma-delimited sequence.) Values
-	// duplicated by other fields in this struct (e.g., ContentLength) are
-	// omitted from Header.
+	// be semantically equivalent to a comma-delimited sequence.) When
+	// Header values are duplicated by other fields in this struct (e.g.,
+	// ContentLength, TransferEncoding, Trailer), the field values are
+	// authoritative.
 	//
 	// Keys in the map are canonicalized (see CanonicalHeaderKey).
 	Header Header