ssh: fix error variable naming convention, add docs

Follow up to CL 96336

Change-Id: I038f3901919c5136273e5df051bc6e958082f830
Reviewed-on: https://go-review.googlesource.com/96415
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

ssh/client_auth_test.go

@@ -614,8 +614,8 @@ func TestClientAuthErrorList(t *testing.T) {
 	for i, e := range authErrs.Errors {
 		switch i {
 		case 0:
-			if e != NoAuthError {
-				t.Fatalf("errors: got error %v, want NoAuthError", e)
+			if e != ErrNoAuth {
+				t.Fatalf("errors: got error %v, want ErrNoAuth", e)
 			}
 		case 1:
 			if e != publicKeyErr {