apm: Refactor outdated cloud apm settings docs (#2027)

Standalone APM (legacy) on ECH should refer to general APM configuration
docs to improve docs maintainability. The actual ECH user settings
involves allowlist and denylist which can be too complicated to
document. Add a generic statement that some settings may be restricted
instead.

The outdated pages are
- https://www.elastic.co/docs/reference/apm/cloud/apm-settings
-
https://www.elastic.co/docs/reference/apm/cloud-enterprise/apm-settings
-
https://www.elastic.co/docs/solutions/observability/apm/configure-apm-server

Fixes elastic/apm-server#13602

---------

Co-authored-by: simitt <[email protected]>

Author: carsonip

reference/apm/cloud-enterprise/apm-settings.md

@@ -19,7 +19,7 @@ Starting in {{stack}} version 8.0, how you change APM settings and the settings
 Standalone APM Server (legacy)
 :   Deployments created prior to {{stack}} version 8.0 are in legacy mode. Upgrading to or past {{stack}} 8.0 does not remove you from legacy mode.
 
-    Check [Edit standalone APM settings (legacy)](#ece-edit-apm-standalone-settings-ece)for information on how to configure Elastic APM in this mode.
+    Check [Edit standalone APM settings (legacy)](#ece-edit-apm-standalone-settings-ece) for information on how to configure Elastic APM in this mode.
 
 
 To learn more about the differences between these modes, or to switch from Standalone APM Server (legacy) mode to {{fleet}}-managed, check [Switch to the Elastic APM integration](/solutions/observability/apm/switch-to-elastic-apm-integration.md).
@@ -64,17 +64,15 @@ If a setting is not supported by Elastic Cloud Enterprise, you get an error mess
 
 ## Example: Enable RUM and increase the rate limit (legacy) [ece_example_enable_rum_and_increase_the_rate_limit_legacy]
 
-When capturing the user interaction with clients with real user monitoring (RUM), particularly for situations with concurrent clients, you can increase the number of times each IP address can send a request to the RUM endpoint. Version 6.5 includes an additional settings for the LRU cache.
-
-For APM Server with RUM agent version 2.x or 3.x:
+When capturing the user interaction with clients with real user monitoring (RUM), particularly for situations with concurrent clients, you can increase the number of times each IP address can send a request to the RUM endpoint.
 
 ```sh
 apm-server:
+  auth:
+    anonymous:
+      event_limit: 3000
   rum:
     enabled: true
-    event rate:
-      limit: 3000
-      lru_size: 5000
 ```