Tell users to contact support for rate limiting issues (#4584)

* Tell users to contact support for rate limiting issues

* Add link to page about opening support case

* Apply edu's feedback on wording

docs/en/serverless/limitations.asciidoc

@@ -5,3 +5,6 @@
 // :keywords: serverless, observability
 
 Currently, the maximum ingestion rate for the Managed Intake Service (APM and OpenTelemetry ingest) is 11.5 MB/s of uncompressed data (roughly 1TB/d uncompressed equivalent). Ingestion at a higher rate may experience rate limiting or ingest failures.
+
+If you believe you are experiencing rate limiting or other ingest-based failures,
+please {estc-welcome}/get-support-help.html[contact Elastic Support] for assistance.