DevDocs corrections part 1

Started reviewing the developer docs, finding lots of things to correct. I'll do more later, but please review this batch first so I know what, if anything, needs to be fixed moving forward - I'm not entirely sure on the markdownish format used.
Actually, [CVE-2012-2459](https://en.bitcoin.it/wiki/CVEs#CVE-2012-2459). Without this clarification, it is possible to have the same merkle root for multiple different merkle trees - one where the last element exists once, and one where there are two of the same element.