Clarify Actuator's endpoint extension support

See gh-47740

Signed-off-by: mipo256 <mikhailpolivakha@email.com>

Author: Mikhail Polivakha

spring-boot-project/spring-boot-docs/src/docs/antora/modules/reference/pages/actuator/endpoints.adoc

@@ -400,7 +400,7 @@ These endpoints are restricted to their respective technologies.
 For example, javadoc:org.springframework.boot.actuate.endpoint.web.annotation.WebEndpoint[format=annotation] is exposed only over HTTP and not over JMX.
 
 You can write technology-specific extensions by using javadoc:org.springframework.boot.actuate.endpoint.web.annotation.EndpointWebExtension[format=annotation] and javadoc:org.springframework.boot.actuate.endpoint.jmx.annotation.EndpointJmxExtension[format=annotation].
-These annotations let you provide technology-specific operations to augment an existing endpoint.
+These annotations let you provide technology-specific operations to augment an existing endpoint. An endpoint may have at most one extension of each type.
 
 Finally, if you need access to web-framework-specific functionality, you can implement servlet or Spring javadoc:org.springframework.stereotype.Controller[format=annotation] and javadoc:org.springframework.web.bind.annotation.RestController[format=annotation] endpoints at the cost of them not being available over JMX or when using a different web framework.