SPARK-5570: No docs stating that `new SparkConf().set("spark.driver.memory", ...) will not work

I've updated documentation to reflect true behavior of this setting in client vs. cluster mode.

Author: Ilya Ganelin <ilya.ganelin@capitalone.com>

Closes #4665 from ilganeli/SPARK-5570 and squashes the following commits:

5d1c8dd [Ilya Ganelin] Added example configuration code
a51700a [Ilya Ganelin] Getting rid of extra spaces
85f7a08 [Ilya Ganelin] Reworded note
5889d43 [Ilya Ganelin] Formatting adjustment
f149ba1 [Ilya Ganelin] Minor updates
1fec7a5 [Ilya Ganelin] Updated to add clarification for other driver properties
db47595 [Ilya Ganelin] Slight formatting update
c899564 [Ilya Ganelin] Merge remote-tracking branch 'upstream/master' into SPARK-5570
17b751d [Ilya Ganelin] Updated documentation for driver-memory to reflect its true behavior in client vs cluster mode