[DOCS] Clarify usage of the enroll Kibana API (elastic#75348)

* [DOCS] Clarify usage of the enroll Kibana API

I had a discussion with @bytebilly about whether users will call the enroll Kibana API on their own, or if it's only intended for internal use. It seems that only Kibana uses this API to verify the enrollment token from Elasticsearch, so I'm adding a note to indicate this usage, along with some explanatory information about when Kibana calls this API.

* Incorporate changes from review feedback

x-pack/docs/en/rest-api/security/enroll-kibana.asciidoc

@@ -6,6 +6,8 @@
 
 Enables a {kib} instance to configure itself for communication with a secured {es} cluster.
 
+NOTE: This API is currently intended for internal use only by {kib}.
+
 [[security-api-kibana-enrollment-request]]
 ==== {api-request-title}
 
@@ -14,8 +16,8 @@ Enables a {kib} instance to configure itself for communication with a secured {e
 [[security-api-kibana-enrollment-desc]]
 ==== {api-description-title}
 
-Use this API to enable a {kib} instance to configure itself for communications
-with an {es} cluster that already has security features enabled.
+{kib} uses this API internally to configure itself for communications with an
+{es} cluster that already has security features enabled.
 
 [[security-api-client-enrollment-examples]]
 ==== {api-examples-title}