Dev Docs: "Not A Specification"

Preview: http://dg0.dtrt.org/en/developer-reference#not-a-specification

Make explicit that the docs are not a specification and never will be:

* Add a new section to the DevRef entitled "Not A Specification".  See preview link above

* In the first sentence of both the DevGuide and the DevRef, say "but [this] is not a specification", with *not a specification* linked to the new section.

* Remove other mentions of specification in the doc

This is inspired by @gmaxwell's recent [BitcoinTalk post about protocol standardization](https://bitcointalk.org/index.php?topic=876020.msg9784326#msg9784326).  @gmaxwell: I'd appreciate it if you could review the new section preview linked above; it's only 5 short paragraphs.
Maybe we could drop "majority", or use "overwhelming majority"? Majority usually means 50%+1. I feel this could be confusing given that forks obey to different specifications and which specification will "win" usually depends on people running the software agreeing on where the fault is or what is the right thing to do.