[SPARK-5836] [DOCS] [STREAMING] Clarify what may cause long-running Spark apps to preserve shuffle files

Clarify what may cause long-running Spark apps to preserve shuffle files

Author: Sean Owen <sowen@cloudera.com>

Closes #6901 from srowen/SPARK-5836 and squashes the following commits:

a9faef0 [Sean Owen] Clarify what may cause long-running Spark apps to preserve shuffle files