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.
Thanks for the link!  Do you think we could more simply say? "Duplicate txids are not permitted."

Or perhaps, "Duplicated txids are not permitted anywhere in the block chain, with the exception of two cases described by BIP30."

This seems to address the problems described by both BIP30 and the linked CVE.