Update threshold-alert.asciidoc (#4349)

Hey, team, I'm sending this pull request because a customer asked about the bars from the preview chart. It was only showing 3 bars instead of 4, as the customer was expecting to see.  However, this is hardcoded and will only show the top 3.

(cherry picked from commit 992facd)

docs/en/observability/threshold-alert.asciidoc

@@ -91,6 +91,8 @@ The shaded area shows the threshold you've set.
 [role="screenshot"]
 image::images/custom-threshold-preview-chart.png[Custom threshold preview chart,75%]
 
+If you use the **Group alerts by** option, the maximum bar size will be 3. It will only show the top 3 fields.
+
 [discrete]
 [[custom-threshold-group-by]]
 == Group alerts by (optional)