This PR adds a section about Hive metastore Parquet table conversion. It documents:

1. Schema reconciliation rules introduced in #5214 (see [this comment] [1] in #5188)
2. Metadata refreshing requirement introduced in #5339

[1]: https://github.com/apache/spark/pull/5188#issuecomment-86531248

Author: Cheng Lian <lian@databricks.com>

Closes #5348 from liancheng/sql-doc-parquet-conversion and squashes the following commits:

42ae0d0 [Cheng Lian] Adds Python `refreshTable` snippet
4c9847d [Cheng Lian] Resorts to SQL for Python metadata refreshing snippet
756e660 [Cheng Lian] Adds Python snippet for metadata refreshing
50675db [Cheng Lian] Addes Hive metastore Parquet table conversion section