[Documentation] run.backburner.DEBUG deprecation warning and guides are wrong

📙 Describe the Issue

There is conflicting (and just plain wrong) information in the guide pages and deprecation warnings regarding how to enable Backburner debugging.

🔬 Minimal Reproduction

I explored this issue in depth here: https://github.com/jacobq/backburner-debug-test#backburner-debug-test

The incorrect deprecation warning message seems to be caused by a limitation of the message template wherein the suggested importKey is derived from the dotKey. That is, run.backburner becomes backburner though it should be _backburner. See https://github.com/jacobq/backburner-debug-test/blob/main/app/app.js#L23-L36

The method specified in the guides produces a deprecation warning in 3.28 and breaks in 4.x.

😕 Actual Behavior

Following the direction of the guides or the deprecation warning lead to incorrect code.

Deprecation message gives wrong suggestion: import { backburner } from '@ember/runloop'

Guides say to do this:

import { run } from '@ember/runloop';

run.backburner.DEBUG = true;

Neither of these works in Ember 4.x.

🤔 Expected Behavior

The guides, deprecation warning, and RFC176 data should all be in agreement and their suggestion should work (not break or produce deprecation warnings).

Deprecation message should say:

Using `run.backburner` has been deprecated. Instead, import the value directly from @ember/runloop:\n\n  import { _backburner } from '@ember/runloop'`

but it is missing the _.

Guides should say to do this:
```
import { _backburner } from '@ember/runloop';
_backburner.DEBUG = true;
```
It would also be good if the guides could state whether or not this is public API, as it does not appear to be listed publicly in the API docs

➕ Additional Context

This may, in fact, be a special case of a larger general problem where the old/global property key is not the same as the import key. For example, Ember.run.hasScheduledTimers -> import { _hasScheduledTimers } from '@ember/runloop'; though I didn't test that.

emberjs / ember.js