Dev Docs: Describe Initial Peer Discovery & DNS Seeds

Preview: http://dg0.dtrt.org/en/developer-guide#p2p-network

* Revise P2P Networking section to provide a more detailed description
  of initial peer discovery and DNS seeds.

* Tweak autocrossref.rb to allow term blacklisting. Then use this new feature
  to prevent cross referencing "address" in "IP address" to the
  definition of Bitcoin P2PKH/P2SH addresses.  This avoids a bunch of
  manual "norefs".

@instagibbs: this revises some of your text; please let me know if you see anything I missed.

Note: this is the first of several expansions I plan to make to the P2P Networking and Operating Mode sections over the next couple weeks.  I'm breaking them up into medium-sized commits to hopefully make reviewing them less painful than my normal giant commits. :-)