[SQL] Update SQL Programming Guide

Author: Michael Armbrust <michael@databricks.com>
Author: Yin Huai <huai@cse.ohio-state.edu>

Closes #2258 from marmbrus/sqlDocUpdate and squashes the following commits:

f3d450b [Michael Armbrust] fix brackets
bea3bfa [Michael Armbrust] Davies suggestions
3a29fe2 [Michael Armbrust] tighten visibility
a71aa36 [Michael Armbrust] Draft of doc updates
52932c0 [Michael Armbrust] Merge remote-tracking branch 'origin/master' into sqlDocUpdate
1e8c849 [Yin Huai] Update the example used for applySchema.
9457c39 [Yin Huai] Update doc.
31ba240 [Yin Huai] Merge remote-tracking branch 'upstream/master' into dataTypeDoc
29bc668 [Yin Huai] Draft doc for data type and schema APIs.