Roland Bless' comment 1

Text is:

""1.  JP MUST connect to its chosen bootstrap node."
it is unclear what "connect" exactly means. This is actually
described in Sec. 11.5:
  "When contacting a bootstrap node, the joining node MUST first form
   the DTLS or TLS connection to the bootstrap node and then sends an
   Attach request over this connection with the destination Node-ID set
   to the joining node's Node-ID."
So my suggestion is to explain it a little but more or to put a forward
reference to section 11.5."

- Add reference to section 11.4.