Add docs on turbine.combineHostPort

docs/src/main/asciidoc/spring-cloud-netflix.adoc

@@ -581,6 +581,8 @@ turbine:
 
 Spring Cloud provides a `spring-cloud-starter-turbine` that has all the dependencies you need to get a Turbine server running. Just create a Spring Boot application and annotate it with `@EnableTurbine`.
 
+NOTE: by default the native Netflix behaviour built into Turbine does _not_ allow multiple processes per host, per cluster (the key to the instance id is the hostname). Spring Cloud generalizes this a bit, allowing the host and port to be used as the key, but only if you set the property `turbine.combineHostPort=true`
+
 === Turbine Stream
 
 In some environments (e.g. in a PaaS setting), the classic Turbine model of pulling metrics from all the distributed Hystrix commands doesn't work. In that case you might want to have your Hystrix commands push metrics to Turbine, and Spring Cloud enables that with messaging. All you need to do on the client is add a dependency to `spring-cloud-netflix-hystrix-stream` and the `spring-cloud-starter-stream-*` of your choice (see Spring Cloud Stream documentation for details on the brokers, and how to configure the client credentials, but it should work out of the box for a local broker).