Specify divergent conversion behavior in dart:js_interop APIs within the docs and not in a note

[srujzs]

Currently, divergent behavior is written in a note e.g. https://api.flutter.dev/flutter/dart-js_interop/ListToJSArray/toJS.html. We've historically treated this as unspecified behavior to signal to users that they shouldn't rely on one set of behavior and have it break when switching to a different compiler.

Instead, we should be explicit about said divergent behavior and make that part of the documentation itself. If users do want to rely on one set of behavior with the caveat that they are not targeting other platforms (e.g. only JS and not Wasm), that should be okay.

This came from the discussion around List.toJS in dart-lang/co19#3307.