net/http/httputil: tell people not to use ClientConn and ServerConn

A very smart developer at Gophercon just asked me to help debug
a problem and I was horrified to learn that he was using httputil's
ClientConn. I forgot ClientConn and ServerConn were even included
in Go 1! They should've been deleted.

Scare people away from using them. The net/http package does
not use them and they're unused, unmaintained and untouched in
4+ years.

LGTM=r
R=r, adg
CC=golang-codereviews
https://golang.org/cl/92790043

src/pkg/net/http/httputil/persist.go

@@ -31,8 +31,8 @@ var errClosed = errors.New("i/o operation on closed connection")
 // i.e. requests can be read out of sync (but in the same order) while the
 // respective responses are sent.
 //
-// ServerConn is low-level and should not be needed by most applications.
-// See Server.
+// ServerConn is low-level and old. Applications should instead use Server
+// in the net/http package.
 type ServerConn struct {
 	lk              sync.Mutex // read-write protects the following fields
 	c               net.Conn
@@ -47,6 +47,9 @@ type ServerConn struct {
 
 // NewServerConn returns a new ServerConn reading and writing c. If r is not
 // nil, it is the buffer to use when reading c.
+//
+// ServerConn is low-level and old. Applications should instead use Server
+// in the net/http package.
 func NewServerConn(c net.Conn, r *bufio.Reader) *ServerConn {
 	if r == nil {
 		r = bufio.NewReader(c)
@@ -221,8 +224,8 @@ func (sc *ServerConn) Write(req *http.Request, resp *http.Response) error {
 // supports hijacking the connection calling Hijack to
 // regain control of the underlying net.Conn and deal with it as desired.
 //
-// ClientConn is low-level and should not be needed by most applications.
-// See Client.
+// ClientConn is low-level and old. Applications should instead use
+// Client or Transport in the net/http package.
 type ClientConn struct {
 	lk              sync.Mutex // read-write protects the following fields
 	c               net.Conn
@@ -238,6 +241,9 @@ type ClientConn struct {
 
 // NewClientConn returns a new ClientConn reading and writing c.  If r is not
 // nil, it is the buffer to use when reading c.
+//
+// ClientConn is low-level and old. Applications should use Client or
+// Transport in the net/http package.
 func NewClientConn(c net.Conn, r *bufio.Reader) *ClientConn {
 	if r == nil {
 		r = bufio.NewReader(c)
@@ -252,6 +258,9 @@ func NewClientConn(c net.Conn, r *bufio.Reader) *ClientConn {
 
 // NewProxyClientConn works like NewClientConn but writes Requests
 // using Request's WriteProxy method.
+//
+// New code should not use NewProxyClientConn. See Client or
+// Transport in the net/http package instead.
 func NewProxyClientConn(c net.Conn, r *bufio.Reader) *ClientConn {
 	cc := NewClientConn(c, r)
 	cc.writeReq = (*http.Request).WriteProxy