oauth2/google: better instructions for the PKCS12 to PEM conversion

Change-Id: Ibcbde1e0131e0a1f90f66978d9b9a7f3aa3dceb6
Reviewed-on: https://go-review.googlesource.com/1660
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

google/example_test.go

@@ -83,9 +83,10 @@ func Example_serviceAccount() {
 		// If you have a p12 file instead, you
 		// can use `openssl` to export the private key into a pem file.
 		//
-		//    $ openssl pkcs12 -in key.p12 -out key.pem -nodes
+		//    $ openssl pkcs12 -in key.p12 -passin pass:notasecret -out key.pem -nodes
 		//
-		// It only supports PEM containers with no passphrase.
+		// The field only supports PEM containers with no passphrase.
+		// The openssl command will convert p12 keys to passphrase-less PEM containers.
 		PrivateKey: []byte("-----BEGIN RSA PRIVATE KEY-----..."),
 		Scopes: []string{
 			"https://www.googleapis.com/auth/bigquery",