go.crypto/ssh: some cleanup

Simplify MarshalAuthorizedKey by using the algoName func.
Make the algoName func be very specific about supported key types in openssh certs.
Generalize some of the commentary that previously mentioned specific key types.

R=agl, dave
CC=golang-dev
https://golang.org/cl/6938067

ssh/certs.go

@@ -11,7 +11,8 @@ import (
 	"time"
 )
 
-// String constants in [PROTOCOL.certkeys] for certificate algorithm names.
+// These constants from [PROTOCOL.certkeys] represent the algorithm names
+// for certificate types supported by this package.
 const (
 	CertAlgoRSAv01      = "ssh-rsa-cert-v01@openssh.com"
 	CertAlgoDSAv01      = "ssh-dss-cert-v01@openssh.com"