Clarify `ns` form rules

[camsaul]

Clarify `ns` form rules

[bbatsov]

I'd be fine to backfill whatever's missing here from "How to ns". PRs welcome (e.g. a PR per bullet point, so it's easier to discuss stuff if there are varying opinions on some topic)!

[seancorfield]

Agreed.

The only ns guideline I feel strongly about is the newline after :require/:import before the list of nses/classes -- which is a style I really dislike, so I'm happy the guide still says "good" for the format without that newline. With automated ns-sorting tools that retain the newline or no newline layout, the argument for the newline (easier sorting) is somewhat moot, IMO.

I'd certainly like to see those "missing" guidelines added here. There are probably other things in Sierra's "Do's and Don'ts" series that might be worth incorporating into the guide...

[seancorfield]

Is anyone volunteering to submit up to five PRs for this? (one for each bullet point)