Scenarios, explain VUs distribute over iters non-uniformly (#1016)

Closes #136

Co-authored-by: Mihail Stoykov <312246+mstoykov@users.noreply.github.com>

src/data/markdown/translated-guides/en/02 Using k6/14 Scenarios/00 Concepts/02 Arrival-rate VU allocation.md

@@ -112,3 +112,9 @@ Some of the times it might make sense to use `maxVUs` include:
 - To determine necessary allocation in first-time tests
 - To add a little "cushion" to the pre-allocated VUs that you expect the test needs
 - In huge, highly distributed tests, in which you need to carefully scale load generators as you increment VUs.
+
+## You can't guarantee which VU runs an iteration
+
+As with all executors, you can't predict the specific VU that an arrival-rate executor uses for a specific iteration.
+As the test runs on, the executor might use some or all of the allocated VUs, even if it never needs the entire allocated number to reach the iteration rate.
+

src/data/markdown/translated-guides/en/02 Using k6/14 Scenarios/01 Executors.md

@@ -9,6 +9,19 @@ want to model to test your services.
 
 Possible values for `executor` are the executor name separated by hyphens.
 
+<Blockquote mod="note" title="VUs might not distribute uniformely over iterations">
+
+For any given scenario, you can't guarantee that a specific VU can run a specific iteration.
+
+With [`SharedArray`](/javascript-api/k6-data/sharedarray/) and [execution context variables](/using-k6/execution-context-variables/), you can map a specific VU to a specific value in your test data.
+So the tenth VU could use the tenth item in your array (or the sixth iteration to the sixth item).
+
+But, you _cannot_ reliably map, for example, the tenth VU to the tenth iteration.
+
+</Blockquote>
+
+
+
 | Name                                                                         | Value                   | Description                                                                                                                                        |
 | ---------------------------------------------------------------------------- | ----------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------- |
 | [Shared iterations](/using-k6/scenarios/executors/shared-iterations)         | `shared-iterations`     | A fixed amount of iterations are<br/> "shared" between a number of VUs.                                                                            |