[api docs] update the docs for dart:convert
[api docs] update the docs for dart:convert

Change-Id: Ia595ff80148327c0d94c8fbf6e295f97796f9371
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/216562
Reviewed-by: Lasse R.H. Nielsen <80295e4ef244a07779f93eaa95d582bad8e316a5@google.com>
Commit-Queue: Lasse R.H. Nielsen <80295e4ef244a07779f93eaa95d582bad8e316a5@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk