clarify doc on configuring apm agent central configuration (#4728)

docs/en/observability/apm/configure/agent-config.asciidoc

@@ -1,41 +1,41 @@
 [[apm-configure-agent-config]]
-= Configure APM agent configuration
+= Configure APM agent central configuration
 
 ++++
-<titleabbrev>APM agent configuration</titleabbrev>
+<titleabbrev>APM agent central configuration</titleabbrev>
 ++++
 
 ****
 image:./binary-yes-fm-yes.svg[supported deployment methods]
 
-APM agent configuration is supported by all APM Server deployment methods.
+APM agent central configuration is supported by all APM Server deployment methods,
+but some options are only supported for APM Server binary users.
 ****
 
-APM agent configuration allows you to fine-tune your APM agents from within the Applications UI.
+APM agent central configuration allows you to fine-tune your APM agents from within the Applications UI.
 Changes are automatically propagated to your APM agents, so there's no need to redeploy your applications.
 
-To learn more about this feature, see <<apm-agent-configuration,APM agent configuration>>.
+To learn more about this feature, see <<apm-agent-configuration,APM agent central configuration>>.
 
-Here's a sample configuration:
+[float]
+== APM agent configuration options
+
+*The following options are only supported for APM Server binary users*.
+
+You can specify APM agent configuration options in the `apm-server.agent.config` section of the
+`apm-server.yml` config file. Here's a sample configuration:
 
 [source,yaml]
 ----
 apm-server.agent.config.cache.expiration: 45s
 apm-server.agent.config.elasticsearch.api_key: TiNAGG4BaaMdaH1tRfuU:KnR6yE41RrSowb0kQ0HWoA
 ----
 
-[float]
-== APM agent configuration options
-
-The following options are only supported for APM Server binary users.
-You can specify these options in the `apm-server.agent.config` section of the
-+apm-server.yml+ config file:
-
 [float]
 [[apm-agent-config-cache]]
 === `apm-server.agent.config.cache.expiration`
 
-When using APM agent configuration, information fetched from {es} will be cached in memory for some time.
+When using APM agent central configuration, information fetched from {es} will be cached in memory for some time.
 Specify the cache expiration time via this setting. Defaults to `30s` (30 seconds).
 
 [float]
@@ -45,14 +45,15 @@ Specify the cache expiration time via this setting. Defaults to `30s` (30 second
 Takes the same options as <<apm-elasticsearch-output,output.elasticsearch>>.
 
 For APM Server binary users and Elastic Agent standalone-managed APM Server,
-APM agent configuration is automatically fetched from {es} using the `output.elasticsearch`
+APM agent central configuration is automatically fetched from {es} using the `output.elasticsearch`
 configuration. If `output.elasticsearch` isn't set or doesn't have sufficient privileges,
 use these {es} options to provide {es} access.
 
 [float]
 === Common problems
 
-You may see either of the following HTTP 403 errors from APM Server when it attempts to fetch APM agent configuration:
+You may see either of the following HTTP 403 errors from APM Server when it attempts to fetch
+the APM agent central configuration:
 
 APM agent log:
 
@@ -71,4 +72,5 @@ rejecting fetch request: no valid elasticsearch config
 This occurs because the user or API key set in either `apm-server.agent.config.elasticsearch` or `output.elasticsearch`
 (if `apm-server.agent.config.elasticsearch` is not set) does not have adequate permissions to read source maps from {es}.
 
-To fix this error, ensure that APM Server has all the required privileges. See <<apm-privileges-agent-central-config-server>> for more details.
+To fix this error, ensure that APM Server has all the required privileges. For more details, refer to
+<<apm-privileges-agent-central-config-server,Create a _central configuration management_ role>>.