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.
BIP 30 is technically obsolete at this point. Actually checking for duplicate txids would have too much overhead. It's better to simply state the fact that it isn't practical to have them.