[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
[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()