Clarify guideline around one-sentence summary in doc comment

[munificent]

1. Use the same comment for both the good and bad examples to show the difference.
2. Explain that part of the motivation is the "inverted pyramid" style of writing.
3. More clearly stress the value users get from doing this: better tooling, quicker skimming, etc.

[Sfshaza]

This issue was moved to dart-lang/site-www#49