Added a bunch of tags.

Also changed parquetFile to take varargs rather than a string followed by varargs.

Author: Reynold Xin <rxin@databricks.com>

Closes #4636 from rxin/df-doc and squashes the following commits:

651f80c [Reynold Xin] Fixed parquetFile in PySpark.
8dc3024 [Reynold Xin] [SQL] Various DataFrame doc changes.