camel-kafka: improve the documentation about consumer streams vs consumers count

We have 2 settings that control the concurrency of the component:

• consumerStreams
• consumerCount

The first one determines the size of the thread pool used for the consumers whereas the second determines the number of concurrent consumer threads.

Our documentation does not make it clear the distinction between these 2 settings and misconfiguring them may lead to degradation of performance on the consumer end.