Self-contained examples with parameter descriptions in PySpark documentation

This JIRA aims to improve PySpark documentation in:

• pyspark
• pyspark.sql
• pyspark.sql.streaming

We should:

• Make the examples self-contained, e.g., https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.pivot.html
• Document Parameters https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.pivot.html#pandas.DataFrame.pivot. There are many API that misses parameters in PySpark, e.g., DataFrame.union

If the size of file is large, e.g., dataframe.py, we should split that down into each subtask, and improve documentation.