net/http: add comment to exported ErrNoCookie and ErrNoLocation values

All other exported errors in net/http are commented.  This change adds
documentation to ErrNoCookie and ErrNoLocation to explain where they are
returned, and why.

Change-Id: I21fa0d070dd35256681ad0714000f238477d4af1
Reviewed-on: https://go-review.googlesource.com/11044Reviewed-by: Russ Cox <rsc@golang.org>

src/net/http/request.go

@@ -243,6 +243,7 @@ func (r *Request) Cookies() []*Cookie {
 	return readCookies(r.Header, "")
 }
 
+// ErrNoCookie is returned by Request's Cookie method when a cookie is not found.
 var ErrNoCookie = errors.New("http: named cookie not present")
 
 // Cookie returns the named cookie provided in the request or

src/net/http/response.go

@@ -93,6 +93,8 @@ func (r *Response) Cookies() []*Cookie {
 	return readSetCookies(r.Header)
 }
 
+// ErrNoLocation is returned by Response's Location method
+// when no Location header is present.
 var ErrNoLocation = errors.New("http: no Location header in response")
 
 // Location returns the URL of the response's "Location" header,