This improves the documentation of the `compute()` function as follows:

  * Instead of making `compute` a top-level constant, we make it a
    function. This allows the generated API docs to show a function
    signature with parameters and their names, making it *much* clearer
    to users what function is being documented.

  * We mention that on web-backends this is running `compute()` on the
    normal eventloop whereas on on native platforms it runs in a
    separate isolate.

  * We mention that callback, message and result have to be sendable
    across isolates. We also mention that they may be copied.

  * We link to both `Isolate.run` & `SendPort.send` for more information.

  * We use `M` for the type of `message` instead the rather confusing
    `Q`.