update dht_sec document

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@9529 a83610d8-ad2a-0410-a6ab-fc0612d85776
merged dht node ID change from RC_0_16

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@9531 a83610d8-ad2a-0410-a6ab-fc0612d85776