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".

_plugins/autocrossref.rb

@@ -71,7 +71,13 @@ require 'yaml'
 		     ## becomes an issue, we can devise a more complex
 		     ## regex
             (?!\w)  ## Don't match inside words
-          /xmi, "[\\&][#{term[1]}]{:.auto-link}")
+          /xmi) {|s|
+              if term[1] == "DO NOT AUTOCROSSREF"
+                  s.gsub(/( |$)/, "<!--noref-->\\&")
+              else
+                  "[#{s}][#{term[1]}]{:.auto-link}"
+              end
+              }
       }
       output.gsub!(/<!--.*?-->/m,'')  ## Remove all HTML comments