Author: Michael Armbrust <michael@databricks.com>

Closes #4642 from marmbrus/docs and squashes the following commits:

d291c34 [Michael Armbrust] python tests
9be66e3 [Michael Armbrust] comments
d56afc2 [Michael Armbrust] fix style
f004747 [Michael Armbrust] fix build
c4a907b [Michael Armbrust] fix tests
42e2b73 [Michael Armbrust] [SQL] Documentation / API Clean-up.