Document server-side gRPC health opt-in behavior

Explain that server-side gRPC health is disabled by default. Update
actuator health example to explicitly enable it.

Signed-off-by: Andrey Litvitski <[email protected]>

Author: therepanic

spring-grpc-docs/src/main/antora/modules/ROOT/pages/health.adoc

@@ -7,6 +7,8 @@ The health service is registered with the gRPC server and a `HealthStatusManager
 
 IMPORTANT: The health service resides in the `io.grpc:grpc-services` library which is marked as `optional` by Spring gRPC. You must add this dependency to your application in order for it to be autoconfigured.
 
+NOTE: Server-side gRPC health is disabled by default. Set `spring.grpc.server.health.enabled=true` to enable it.
+
 == Actuator Health
 When Spring Boot Actuator is added to your project and the {spring-boot-docs}/actuator/endpoints.html#actuator.endpoints.health[Health endpoint] is available, the framework will automatically periodically update the health status of a configured list of Spring Boot {spring-boot-docs}/actuator/endpoints.html#actuator.endpoints.health.auto-configured-health-indicators[health indicators], including any ({spring-boot-docs}/actuator/endpoints.html#actuator.endpoints.health.writing-custom-health-indicators[custom indicators]).
 By default, the aggregate status of the individual indicators is also used to update the overall server status (`""`).
@@ -19,6 +21,7 @@ spring:
   grpc:
     server:
       health:
+        enabled: true
         actuator:
           health-indicator-paths:
             - db