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

This pull request propose to merge into the branch for #393 several small changes based on feedback from @luke-jr and @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!