Update real-user-monitoring-rum.md

Add clarification that APM RUM is not available for Serverless

Author: maggieghamry

solutions/observability/apm/apm-agents/real-user-monitoring-rum.md

@@ -3,6 +3,7 @@ mapped_pages:
   - https://www.elastic.co/guide/en/observability/current/apm-rum.html
 applies_to:
   stack: all
+  serverless: unavailable
 products:
   - id: observability
   - id: apm
@@ -16,3 +17,5 @@ Unlike Elastic APM backend agents which monitor requests and responses, the RUM
 
 You will be able to measure metrics such as "Time to First Byte", `domInteractive`, and `domComplete` which helps you discover performance issues within your client-side application as well as issues that relate to the latency of your server-side application.
 
+::::{note}
+Elastic APM RUM is *not* compatible with [{{serverless-full}}](docs-content://deploy-manage/deploy/elastic-cloud/serverless.md).