Small Changes To Devel Doc Based On Comments

[harding]

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!

[harding]

Added another small change to this pull, this one suggested by @TierNolan. See c59b8e0 and saivann#130 for details. Thanks!

[saivann]

@harding LGTM. Unless someone adds a comment until then, this pull request will be merged into the main pull request today at 18:00:00 +0000 .