go.crypto/ssh: Miscellaneous changes up for discussion.

Export key and certificate algorithm names.
Switch from string literals over to using the constants for any key/cert algorithm references.
Make URL references visible in the godoc web display.
Standardize url reference names with surrounding [].

R=dave, agl, jonathan.mark.pittman
CC=golang-dev
https://golang.org/cl/6944047

ssh/messages.go

@@ -13,8 +13,7 @@ import (
 )
 
 // These are SSH message type numbers. They are scattered around several
-// documents but many were taken from
-// http://www.iana.org/assignments/ssh-parameters/ssh-parameters.xml#ssh-parameters-1
+// documents but many were taken from [SSH-PARAMETERS].
 const (
 	msgDisconnect     = 1
 	msgIgnore         = 2