[DOC][MINOR] Kafka doc: breakup into lines

## Before

![before](https://cloud.githubusercontent.com/assets/15843379/20340231/99b039fe-ac1b-11e6-9ba9-b44582427459.png)

## After

![after](https://cloud.githubusercontent.com/assets/15843379/20340236/9d5796e2-ac1b-11e6-92bb-6da40ba1a383.png

)

Author: Liwei Lin <lwlin7@gmail.com>

Closes #15903 from lw-lin/kafka-doc-lines.

(cherry picked from commit 3e01f128)
Signed-off-by: Sean Owen <sowen@cloudera.com>