[SPARK-3074] [PySpark] support groupByKey() with single huge key

This patch change groupByKey() to use external sort based approach, so it can support single huge key.

For example, it can group by a dataset including one hot key with 40 millions values (strings), using 500M memory for Python worker, finished in about 2 minutes. (it will need 6G memory in hash based approach).

During groupByKey(), it will do in-memory groupBy first. If the dataset can not fit in memory, then data will be partitioned by hash. If one partition still can not fit in memory, it will switch to sort based groupBy(). 
Actually, I double-checked and this is just a documentation change, since the previous version of groupByKey() behaves the same way.