Copyediting and recommended edits

Don't accept this PR yet unless you're 100% fine with the changes. I might have more edits and so might you. I fixed some small and obvious things and had some recommendations for re-wording as well.

Check out the new "rendered" prose diff view instead of the source diff – it's a [great new feature just added this week on Github](https://github.com/blog/1784-rendered-prose-diffs) and allows for much more careful review of writing changes.