wgengine/magicsock: update comments on what implements conn.Bind

The comment still said *magicsock.Conn implemented wireguard-go conn.Bind.
That wasn't accurate anymore.

A doc #cleanup.

Change-Id: I7fd003b939497889cc81147bfb937b93e4f6865c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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