Add warning to the group API about asynchronize code (#1011)

Co-authored-by: Matt Dodson <47385188+MattDodsonEnglish@users.noreply.github.com>
Co-authored-by: na-- <n@andreev.sh>

src/data/markdown/docs/02 javascript api/02 k6/group- name- fn -.md

@@ -17,6 +17,19 @@ Run code inside a group. Groups are used to organize results in a test.
 | ---- | ------------------------- |
 | any  | The return value of _fn_. |
 
+<Blockquote mod="warning" title="Working with async functions">
+
+Avoid using `group` with async functions or asynchronous code.
+If you do, k6 might apply tags in a way that is unreliable or unintuitive.
+
+If you start promise chains or even use `await` within `group`, some code within the group will be waited for and tagged with the proper `group` tag, but others won't be.
+
+To avoid confusion, `async` functions are forbidden as `group()` arguments. This still lets users make and chain promises within a group, but doing so is unsupported and not recommended.
+
+For more information, refer to [k6 #2728](https://github.com/grafana/k6/issues/2728), which tracks possible solutions and provides detailed explanations.
+
+</Blockquote>
+
 ### Example
 
 <CodeGroup labels={[]}>