improve PR description verifier

[estroz]

The PR description verifier tool checks if a description is "descriptive" enough, i.e. >= 2 lines. This is kinda brittle because one long line is considered less descriptive than 2 short lines. Instead, I propose that a minimum character length (say, 80) be used to quantify descriptiveness.

Additionally there should be a way to override this check and have it pass on special PRs, ex. cherry picks, that typically have short descriptions.

/kind feature