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.
The "coinbase" refers to the scriptSig in the generation transaction. I am probably at fault for using "coinbasetxn" in GBT, but I think it's best to use clearer terminology, even if we have a ("coinbase transaction") in quotes once to avoid confusion.