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

Updates #cleanup

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

derp/xdp/headers/update.go

@@ -1,6 +1,8 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
+// The update program fetches the libbpf headers from the libbpf GitHub repository
+// and writes them to disk.
 package main
 
 import (

derp/xdp/xdp.go

@@ -1,6 +1,7 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
+// Package xdp contains the XDP STUN program.
 package xdp
 
 // XDPAttachFlags represents how XDP program will be attached to interface. This