Update endpoint enablement description to describe availability

See gh-20799

Author: ericus20

spring-boot-project/spring-boot-docs/src/main/asciidoc/production-ready-features.adoc

@@ -49,7 +49,7 @@ Spring Boot includes a number of built-in endpoints and lets you add your own.
 For example, the `health` endpoint provides basic application health information.
 
 Each individual endpoint can be <<production-ready-endpoints-enabling-endpoints, enabled or disabled>>.
-This controls whether or not the endpoint is created and its bean exists in the application context.
+An endpoint that is enabled is said to be available if it is exposed (made remotely accessible) over HTTP or JMX.
 To be remotely accessible an endpoint also has to be <<production-ready-endpoints-exposing-endpoints, exposed via JMX or HTTP>>.
 Most applications choose HTTP, where the ID of the endpoint along with a prefix of `/actuator` is mapped to a URL.
 For example, by default, the `health` endpoint is mapped to `/actuator/health`.