ipn/ipnlocal: use context.CancelFunc type for doc clarity

Using context.CancelFunc as the type (instead of func()) answers
questions like whether it's okay to call it multiple times, whether
it blocks, etc. And that's the type it actually is in this case.

Updates #cleanup

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -163,7 +163,7 @@ type watchSession struct {
 	ch        chan *ipn.Notify
 	owner     ipnauth.Actor // or nil
 	sessionID string
-	cancel    func() // call to signal that the session must be terminated
+	cancel    context.CancelFunc // to shut down the session
 }
 
 // LocalBackend is the glue between the major pieces of the Tailscale