[SPARK-17853][STREAMING][KAFKA][DOC] make it clear that reusing group.id is bad
[SPARK-17853][STREAMING][KAFKA][DOC] make it clear that reusing group.id is bad

## 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.

(cherry picked from commit c264ef9b1918256a5018c7a42a1a2b42308ea3f7)
Signed-off-by: Reynold Xin <c9d35a384bc579d1c6c8c055f2a2b5c04e11b521@databricks.com>

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