all: add test for package comments, fix, add comments as needed

Updates #cleanup

Change-Id: Ic4304e909d2131a95a38b26911f49e7b1729aaef
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/wgengine.go

@@ -1,6 +1,7 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
+// Package wgengine provides the Tailscale WireGuard engine interface.
 package wgengine
 
 import (

wgengine/winnet/winnet.go

@@ -3,6 +3,7 @@
 
 //go:build windows
 
+// Package winnet contains Windows-specific networking code.
 package winnet
 
 import (