[Flow analysis] Documentation cleanup.

Inspired by some recent code reviews from Lasse, I decided to make
some improvements to flow analysis documentation to bring it more in
line with the Dart style guide.

Change-Id: Ibbda802055d7e17429806b2a4e608005bf761a69
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/433380
Reviewed-by: Lasse Nielsen <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441