[SPARK-4258][SQL][DOC] Documents spark.sql.parquet.filterPushdown

Documents `spark.sql.parquet.filterPushdown`, explains why it's turned off by default and when it's safe to be turned on.

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Author: Cheng Lian <lian@databricks.com>

Closes #3440 from liancheng/parquet-filter-pushdown-doc and squashes the following commits:

2104311 [Cheng Lian] Documents spark.sql.parquet.filterPushdown

(cherry picked from commit 5db8dcaf494e0dffed4fc22f19b0334d95ab6bfb)
Signed-off-by: Michael Armbrust <michael@databricks.com>
[SQL] add @group tab in limit() and count()

group tab is missing for scaladoc

Author: Jacky Li <jacky.likun@gmail.com>

Closes #3458 from jackylk/patch-7 and squashes the following commits:

0121a70 [Jacky Li] add @group tab in limit() and count()

(cherry picked from commit bafee67ebad01f7aea2cd393a70b57eb8345eeb0)
Signed-off-by: Michael Armbrust <michael@databricks.com>
[SPARK-4358][SQL] Let BigDecimal do checking type compatibility

Remove hardcoding max and min values for types. Let BigDecimal do checking type compatibility.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #3208 from viirya/more_numericLit and squashes the following commits:

e9834b4 [Liang-Chi Hsieh] Remove byte and short types for number literal.
1bd1825 [Liang-Chi Hsieh] Fix Indentation and make the modification clearer.
cf1a997 [Liang-Chi Hsieh] Modified for comment to add a rule of analysis that adds a cast.
91fe489 [Liang-Chi Hsieh] add Byte and Short.
1bdc69d [Liang-Chi Hsieh] Let BigDecimal do checking type compatibility.

(cherry picked from commit b57365a1ec89e31470f424ff37d5ebc7c90a39d8)
Signed-off-by: Michael Armbrust <michael@databricks.com>
[SPARK-4650][SQL] Supporting multi column support in countDistinct function like count(distinct c1,c2..) in Spark SQL

Supporting multi column support in countDistinct function like count(distinct c1,c2..) in Spark SQL

Author: ravipesala <ravindra.pesala@huawei.com>
Author: Michael Armbrust <michael@databricks.com>

Closes #3511 from ravipesala/countdistinct and squashes the following commits:

cc4dbb1 [ravipesala] style
070e12a [ravipesala] Supporting multi column support in count(distinct c1,c2..) in Spark SQL

(cherry picked from commit 6a9ff19dc06745144d5b311d4f87073c81d53a8f)
Signed-off-by: Michael Armbrust <michael@databricks.com>
[SPARK-4658][SQL] Code documentation issue in DDL of datasource API

Author: ravipesala <ravindra.pesala@huawei.com>

Closes #3516 from ravipesala/ddl_doc and squashes the following commits:

d101fdf [ravipesala] Style issues fixed
d2238cd [ravipesala] Corrected documentation

(cherry picked from commit bc353819cc86c3b0ad75caf81b47744bfc2aeeb3)
Signed-off-by: Michael Armbrust <michael@databricks.com>
[SQL] Minor fix for doc and comment

Author: wangfei <wangfei1@huawei.com>

Closes #3533 from scwf/sql-doc1 and squashes the following commits:

962910b [wangfei] doc and comment fix

(cherry picked from commit 7b79957879db4dfcc7c3601cb40ac4fd576259a5)
Signed-off-by: Michael Armbrust <michael@databricks.com>