## What changes were proposed in this pull request?

Documentation fix to make it clear that reusing group id for different streams is super duper bad, just like it is with the underlying Kafka consumer.

## How was this patch tested?

I built jekyll doc and made sure it looked ok.

Author: cody koeninger <cody@koeninger.org>

Closes #15442 from koeninger/SPARK-17853.