Issue210: Update / Improve comment for 'AccountCredentials'.

Update the docuemntation for AccountCredential to make it clear
what the parts of the proto message mean, with respect to a normal
openssh format ssh public key.

credentialz/credentialz.proto

@@ -435,6 +435,9 @@ enum KeyType {
 
 message AccountCredentials {
   message AuthorizedKey {
+    // AuthorizedKey is an ssh key/idenity. The basic openssh format follows:
+    // ecdsa-sha2-nistp521 AAAkeystuffhere [email protected]
+    //   key_type         authorized_key    description
     bytes authorized_key = 1;
     // Options specified for this authorized key.
     repeated Option options = 2;