all: clean up some Deprecated comments

Change-Id: Ie801fe6a2883d79229ee2955e26948c1b4964802
Reviewed-on: https://go-review.googlesource.com/122496Reviewed-by: Russ Cox <rsc@golang.org>

src/image/jpeg/reader.go

@@ -89,7 +89,8 @@ var unzig = [blockSize]int{
 	53, 60, 61, 54, 47, 55, 62, 63,
 }
 
-// Deprecated: Reader is deprecated.
+// Deprecated: Reader is not used by the image/jpeg package and should
+// not be used by others. It is kept for compatibility.
 type Reader interface {
 	io.ByteReader
 	io.Reader

src/net/http/request.go

@@ -65,11 +65,19 @@ var (
 	// request's Content-Type is not multipart/form-data.
 	ErrNotMultipart = &ProtocolError{"request Content-Type isn't multipart/form-data"}
 
-	// Deprecated: ErrHeaderTooLong is not used.
+	// Deprecated: ErrHeaderTooLong is no longer returned by
+	// anything in the net/http package. Callers should not
+	// compare errors against this variable.
 	ErrHeaderTooLong = &ProtocolError{"header too long"}
-	// Deprecated: ErrShortBody is not used.
+
+	// Deprecated: ErrShortBody is no longer returned by
+	// anything in the net/http package. Callers should not
+	// compare errors against this variable.
 	ErrShortBody = &ProtocolError{"entity body too short"}
-	// Deprecated: ErrMissingContentLength is not used.
+
+	// Deprecated: ErrMissingContentLength is no longer returned by
+	// anything in the net/http package. Callers should not
+	// compare errors against this variable.
 	ErrMissingContentLength = &ProtocolError{"missing ContentLength in HEAD response"}
 )
 

src/net/http/server.go

@@ -51,7 +51,9 @@ var (
 	// declared.
 	ErrContentLength = errors.New("http: wrote more than the declared Content-Length")
 
-	// Deprecated: ErrWriteAfterFlush is no longer used.
+	// Deprecated: ErrWriteAfterFlush is no longer returned by
+	// anything in the net/http package. Callers should not
+	// compare errors against this variable.
 	ErrWriteAfterFlush = errors.New("unused")
 )