[SPARK-4771][Docs] Document standalone cluster supervise mode

tdas looks like streaming already refers to the supervise mode. The link from there is broken though.

Author: Andrew Or <andrew@databricks.com>

Closes #3627 from andrewor14/document-supervise and squashes the following commits:

9ca0908 [Andrew Or] Wording changes
2b55ed2 [Andrew Or] Document standalone cluster supervise mode