Non-content edits to pills 9-14

[peter-mlabs]

This PR (and future PRs I'll be submitting in the same vein) is focused on improving grammar, voicing, stylistic consistency, and readability. As a relatively inexperienced Nix user, I don't have much to add in terms of content or technical commentary; I'd simply like to improve the quality of the existing documentation.

I've found that the earlier pills have a slightly different tone than the later pills -- some of which almost seems like it can be attributed to different authors -- and that the writing style becomes weaker as the material grows in complexity. As such, I only committed to undertaking this editing starting at pill #9 (the point at which I felt my own comprehension beginning to suffer), and I plan on editing as I go through the remaining pills. Once I'm finished, I'll try to go back and revise from the beginning.