Fix documentation for kafka streams concurrency on binder level
Fix documentation for kafka streams concurrency on binder level

The property spring.cloud.stream.binder.configuration.num.stream.threads does not work and is silently ignored.

The property spring.cloud.stream.kafka.streams.binder.configuration.num.stream.threads works fine and  is already covered by tests at MultipleFunctionsInSameAppTests#125

resolves #987

spring-cloud/spring-cloud-stream,spring-cloud/spring-cloud-stream,spring-cloud/spring-cloud-stream,garyrussell/spring-cloud-stream,garyrussell/spring-cloud-stream,garyrussell/spring-cloud-stream