Small Changes To Devel Doc Based On Comments By @luke-jr & @mikehearn

_includes/guide_transactions.md:

* DOS expanded to "denial of service" to improve readability and avoid
  conflation with Disk Operating System. Change based on feedback from
  @luke-jr. Thanks!

_includes/guide_mining.md:

* Dropped assertion that `getblocktemplate` can't reuse an established
  socket. Change based on feedback from @luke-jr. Thanks!

_includes/guide_contracts.md:

* Dropped opening sentences to Contracts section, which were a holdover
  from when contracts was a subsection of Transactions. New opening
  sentence is now similar to the summary sentences which open all other
  sections. Change based on feedback from @mikehearn. Thanks!

* Deleted USA-centric example from second paragraph and merged remaining
  parts of the first two paragraphs into a single opening paragraph with
  no example. Change based on feedback from @mikehearn. Thanks!

* Removed mention of placeholder byte from multisig example. Change
  based on feedback from @mikehearn. Thanks!