[docs] BytesBuilder docs reference pre-class-modifiers statement

[navaronbracke]

I was looking into implementing an algorithm using a BytesBuilder.
Having never used it before, I had a look in its docs and noticed a statement that should be outdated with the introduction of class modifiers.

The statement at
https://github.com/dart-lang/sdk/blob/main/sdk/lib/internal/bytes_builder.dart#L28-L30
contradicts what the class modifiers for Uint8List actually enforce in https://github.com/dart-lang/sdk/blob/main/sdk/lib/typed_data/typed_data.dart#L866-L868

See also the doc comment in Uint8List, which states that implementing / extending should be impossible.

[lrhn]

Well spotted. I'm not actually sure growing Uint8Lists have ever existed. The text referred to Uint8Buffer from package:typed_data which doesn't actually implement Uint8List, it just has a similar interface.
So the text may always have been wrong. Mea culpa.