SPARK-4040. Update documentation to exemplify use of local (n) value, fo...

This is a minor docs update which helps to clarify the way local[n] is used for streaming apps.

Author: jay@apache.org <jayunit100>

Closes #2964 from jayunit100/SPARK-4040 and squashes the following commits:

35b5a5e [jay@apache.org] SPARK-4040: Update documentation to exemplify use of local (n) value.

(cherry picked from commit 868cd4c3ca11e6ecc4425b972d9a20c360b52425)
Signed-off-by: Matei Zaharia <matei@databricks.com>