net: document OpError

Fixes #4797

R=adg, rsc
CC=golang-dev
https://golang.org/cl/7300099

src/pkg/net/net.go

@@ -276,10 +276,22 @@ type Listener interface {
 
 var errMissingAddress = errors.New("missing address")
 
+// OpError is the error type usually returned by functions in the net
+// package. It describes the operation, network type, and address of
+// an error.
 type OpError struct {
+	// Op is the operation which caused the error, such as
+	// "read" or "write".
 	Op string
+
+	// Net is the network type on which this error occurred,
+	// such as "tcp" or "udp6".
 	Net string
+
+	// Addr is the network address on which this error occurred.
 	Addr Addr
+
+	// Err is the error that occurred during the operation.
 	Err error
 }