go/doc, godoc: don't shadow receiver

Fixes #2737.

R=bradfitz
CC=golang-dev
https://golang.org/cl/5553062

src/pkg/go/doc/doc.go

@@ -36,7 +36,7 @@ type Value struct {
 type Method struct {
 	*Func
 	// TODO(gri) The following fields are not set at the moment. 
-	Recv  *Type // original receiver base type
+	Origin *Type // original receiver base type
 	Level  int   // embedding level; 0 means Func is not embedded
 }