[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.
[SPARK-3408] Fixed Limit operator so it works with sort-based shuffle.

Author: Reynold Xin <rxin@apache.org>

Closes #2281 from rxin/sql-limit-sort and squashes the following commits:

1ef7780 [Reynold Xin] [SPARK-3408] Fixed Limit operator so it works with sort-based shuffle.
[SPARK-3415] [PySpark] removes SerializingAdapter code

This code removes the SerializingAdapter code that was copied from PiCloud

Author: Ward Viaene <ward.viaene@bigdatapartnership.com>

Closes #2287 from wardviaene/feature/pythonsys and squashes the following commits:

5f0d426 [Ward Viaene] SPARK-3415: modified test class to do dump and load
5f5d559 [Ward Viaene] SPARK-3415: modified test class name and call cloudpickle.dumps instead using StringIO
afc4a9a [Ward Viaene] SPARK-3415: added newlines to pass lint
aaf10b7 [Ward Viaene] SPARK-3415: removed references to SerializingAdapter and rewrote test
65ffeff [Ward Viaene] removed duplicate test
a958866 [Ward Viaene] SPARK-3415: test script
e263bf5 [Ward Viaene] SPARK-3415: removes legacy SerializingAdapter code