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.