ssh: document how to unmarshal public keys

Fixes golang/go#21491.

Change-Id: I5c720f1f6c6acfa7f4cd50920f65db7e57ba3792
Reviewed-on: https://go-review.googlesource.com/95115
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

ssh/keys.go

@@ -276,7 +276,8 @@ type PublicKey interface {
 	Type() string
 
 	// Marshal returns the serialized key data in SSH wire format,
-	// with the name prefix.
+	// with the name prefix. To unmarshal the returned data, use
+	// the ParsePublicKey function.
 	Marshal() []byte
 
 	// Verify that sig is a signature on the given data using this