[Serverless][Security]: Edit highlighted fields in alert flyout (#1540)

Contributes to #1235 by
updating Serverless docs for the Highlighted fields section in the alert
details flyout.

Preview: [View alert details |
Investigation](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/1540/solutions/security/detect-and-alert/view-detection-alert-details#investigation-section)
- Added a tip after the description for the Highlighted fields table
that lets users now how to quickly add/remove custom highlighted fields
from the rule.

**Corresponding PRs**
- 8.19: elastic/security-docs#6861
- 9.x: TBD

Author: nastasha-solomon

solutions/security/detect-and-alert/view-detection-alert-details.md

@@ -136,6 +136,9 @@ The Investigation section provides the following information:
 
 * **Highlighted fields**: Shows relevant fields for the alert and any [custom highlighted fields](/solutions/security/detect-and-alert/create-detection-rule.md#rule-ui-advanced-params) you added to the rule. Custom highlighted fields with values are added to this section. Those without values aren’t added.
 
+    ::::{tip}
+    In {{serverless-short}}, you can quickly add and remove custom highlighted fields from the rule by clicking **Add field** in the Highlighted fields table. 
+    ::::
 
 ## Visualizations [visualizations-section]