[SPARK-5836] [DOCS] [STREAMING] Clarify what may cause long-running Spark apps...

[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

docs/programming-guide.md

@@ -1144,9 +1144,11 @@ generate these on the reduce side. When data does not fit in memory Spark will s
 to disk, incurring the additional overhead of disk I/O and increased garbage collection.
 
 Shuffle also generates a large number of intermediate files on disk. As of Spark 1.3, these files
-are not cleaned up from Spark's temporary storage until Spark is stopped, which means that
-long-running Spark jobs may consume available disk space. This is done so the shuffle doesn't need
-to be re-computed if the lineage is re-computed. The temporary storage directory is specified by the
+are preserved until the corresponding RDDs are no longer used and are garbage collected. 
+This is done so the shuffle files don't need to be re-created if the lineage is re-computed. 
+Garbage collection may happen only after a long period time, if the application retains references 
+to these RDDs or if GC does not kick in frequently. This means that long-running Spark jobs may 
+consume a large amount of disk space. The temporary storage directory is specified by the
 `spark.local.dir` configuration parameter when configuring the Spark context.
 
 Shuffle behavior can be tuned by adjusting a variety of configuration parameters. See the