Rephrase "Returns a stream" docs (dart-lang/stream_transform#149)

Following up on a recent PR comment about this phrasing in the docs for
`buffer` and `sample`. Rewrite all doc headers that started with
"Returns a stream" to instead focus on the action on the stream.

Reference `debounce` and `audit` from the `throttle` docs.

Move the paragraphs with comparisons to `throttle` and `debounce` to a
"See also" section in the `audit` docs.

Replace some "the original stream" and "the source stream" with "this
stream".

Where some comments had to reflow anyway, separate sentences onto their
own lines to reduce churn in the future in any sentence changes.

Remove some unnecessary `Stream.` prefixes in dartdoc references.
11 files changed
tree: 7e04ddfc7ae369066e4cff8b0c729166bbd9ef6d
  1. pkgs/