This PR only applies to master branch (1.5.0-SNAPSHOT) since it references `org.apache.parquet` classes which only appear in Parquet 1.7.0.

Author: Cheng Lian <lian@databricks.com>

Closes #6683 from liancheng/output-committer-docs and squashes the following commits:

b4648b8 [Cheng Lian] Removes spark.sql.sources.outputCommitterClass as it's not a public option
ee63923 [Cheng Lian] Updates docs and comments of data sources and Parquet output committer options