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.
I like the additional detail, but I have some concerns:

* The phrasing of the first sentence hasn't changed, so I still have a hard time parsing it.

* A paragraph or other extended description here is suboptimal as the next paragraph starts with "For example".  The example is of merkle tree creation, not checking for block hash collision.

My suggestions:

* Move the new paragraph down to line 225 (in the original numbering) after the `{% autocrossref %}`.

* Rephrase.  Here's a modified version of the text I suggested earlier: "Note: two identical txids cannot be paired together because this would produce the same hash as a single txid being paired with a copy of itself. Since it is impractical to have separate transactions with identical txids, this does not impose a burden on honest programs but can prevent bugs such as CVE-2012-2459 from damaging the network."