Add "overwrite" setting info to Write to MongoDB page

Users should know that when you set the option to "overwrite" it will not re-create the exact collection in these scenarios (i.e., time series collection, sharded collection, etc).

Please add this information/set the expectation on https://www.mongodb.com/docs/spark-connector/current/write-to-mongodb/ 

Also see SPARK-364 and the comments there for more information.