Dev Docs: Describe Initial Peer Discovery & DNS Seeds

* Revise P2P Networking section to provide a more detailed description
  of initial peer discovery and DNS seeds.

* Tweak autocrossref.rb to allow term blacklisting. 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".
Describe Hardcoded Peers & BCCore Command-Line Fallbacks

Suggested by @gmaxwell (thanks!)
Merge branch 'peer-discovery' (pull #617)
Treat Mainnet And Testnet As Common Nouns, Lowercasing When Appropriate

Suggested by @saivann (thanks!)