Clean up Map#operator[] documentation
Clean up Map#operator[] documentation

New PR for https://github.com/dart-lang/sdk/pull/32714 to get a squashed view.
Thanks @burkemw3.

Closes #32746
https://github.com/dart-lang/sdk/pull/32746

GitOrigin-RevId: ad3342fc027505abb9dcd0723279bbd276bf6d8a
Change-Id: I00905dded9de083350051a36657b928454b0a9fe
Reviewed-on: https://dart-review.googlesource.com/49101
Reviewed-by: Lasse R.H. Nielsen <80295e4ef244a07779f93eaa95d582bad8e316a5@google.com>
Commit-Queue: Lasse R.H. Nielsen <80295e4ef244a07779f93eaa95d582bad8e316a5@google.com>

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