coroutine_execution_contexts.md: Add a simple example to clarify what the main usage pattern is

topics/basics/architectural_overview/coroutines/coroutine_execution_contexts.md

@@ -51,9 +51,21 @@ Therefore, if their behavior is required, [switch to a blocking context](#suspen
 - [`ProgressStep`](%gh-ic%/platform/util/progress/src/impl/ProgressStep.kt) - a step-based progress reporting (see its KDoc for details)
 - [`RawProgressReporter`](%gh-ic%/platform/util/progress/src/RawProgressReporter.kt) - a raw text, details, and fraction reporting (invoked via [`reportRawProgress()`](%gh-ic%/platform/util/progress/src/steps.kt))
 
-Any [`report*Progress()`](%gh-ic%/platform/util/progress/src/steps.kt) function should be used inside [`with*Progress()` or `runWithModalProgressBlocking()`](%gh-ic%/platform/progress/shared/src/tasks.kt).
+Any [`report*Progress()`](%gh-ic%/platform/util/progress/src/steps.kt) function must be used inside [`withBackgroundProgress()`, `withModalProgress()`, or `runWithModalProgressBlocking()`](%gh-ic%/platform/progress/shared/src/tasks.kt).
 Otherwise, if there is no reporter in the context, using `report*Progress()` will have no effect.
 
+Example:
+
+```kotlin
+withBackgroundProgress(...) { // or withModalProgress/runWithModalProgressBlocking
+  // ...
+  reportProgress { reporter -> // or another report*Progress
+    // do tasks and report progress
+  }
+  // ...
+}
+```
+
 ### Switching to Other Contexts
 {#suspending-context-switching-to-other-contexts}