Mention Bitcoin's RPC Hash Byte Order

Preview: http://dg1.dtrt.org/en/developer-reference#hash-byte-order

* Add a short subsection about the different byte orders used with
  hashes in Bitcoin Core and other software.

* Re-word some text in other sections to mention the byte order
  differences

@SergioDemianLerner do you think this describes the situation sufficiently well for other people to figure out what's going on?
Fixed.  Thanks!