Update ~48 doc comments to have one-sentence summaries

From Effective Dart: Documentation:
* https://www.dartlang.org/guides/language/effective-dart/documentation#do-start-doc-comments-with-a-single-sentence-summary
* https://www.dartlang.org/guides/language/effective-dart/documentation#do-separate-the-first-sentence-of-a-doc-comment-into-its-own-paragraph

Change-Id: Ie49b2e6a2cad5c1d01a6cd625f91f89e4fcc39ce
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/69260
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

Author: srawlins