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/doc.go

@@ -10,5 +10,10 @@ family of application protocols. The most typical application level
 protocol is a remote shell and this is specifically implemented.  However,
 the multiplexed nature of SSH is exposed to users that wish to support
 others.
+
+References:
+  [PROTOCOL.certkeys]: http://www.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/PROTOCOL.certkeys
+  [PROTOCOL.agent]:    http://www.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/PROTOCOL.agent
+  [SSH-PARAMETERS]:    http://www.iana.org/assignments/ssh-parameters/ssh-parameters.xml#ssh-parameters-1
 */
 package ssh