Restore recommendation to use small structs to Dos-and-Don'ts page.

This was lost in the move to markdown.

Also rewrites the make_unique<> section (for brevity and to more explicitly
recommend the "auto t = std::make_unique<T>" idiom) and the comment style
section (for brevity and clarity), and makes some other trivial edits.

I can split these last into other CLs if need be, just figured it'd be
easier to review-at-once.

Bug: none
Change-Id: I6b38e04b8b8d4a354f3704c2201604fab1f35d0b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1512307
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#639716}
1 file changed