Clean up HttpClient docs
Clean up HttpClient docs

In particular, fix typos, and update many comments to begin with a one-sentence
summary, for better readability at api.dartlang.org.

Fixes #31608, #29460, #31538, #31449, #31505

Bug: https://github.com/dart-lang/sdk/issues/31608
Bug: https://github.com/dart-lang/sdk/issues/29460
Bug: https://github.com/dart-lang/sdk/issues/31538
Bug: https://github.com/dart-lang/sdk/issues/31449
Bug: https://github.com/dart-lang/sdk/issues/31505
Change-Id: If00726ef3636479f0613c91e48ff9f0e24b12502
Reviewed-on: https://dart-review.googlesource.com/30120
Commit-Queue: Samuel Rawlins <5cf2cac5eec2ad6d0eb2b0f4ff6af9a590ce2bf6@google.com>
Reviewed-by: Zach Anderson <9f8b31e960a68dade71618f1ff6da6e23a1edfc9@google.com>

dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk