Update doc comments for dart:js

Fixes flutter#38280, Fixes flutter#24576

- Add "Low level" to the library doc comment and add a sentence about
  preferring to use `package:js`.
- Add doc comments on `allowInterop` and `allowInteropCaptureThis`.
- Remove the docs that discuss what dart2js "may" do in the future.
- Tweak some comments on classes to use a noun over a verb - "A proxy
  on" over "Proxies".

Cleanup:

- Use `///` consistently.
- Ensure the docs are identical between the two copies of the file, this
  minimizes the diff.

Change-Id: I7f904d77b2d9dd537201dd53a29d7ef74289aa86
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/116555
Reviewed-by: Kathy Walrath <kathyw@google.com>
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Commit-Queue: Nate Bosch <nbosch@google.com>