Docs for SERVER-33538: mapReduce "replace" on a sharded output collection can lead to UUIDCatalog inconsistencies

Description:

If the out field for mapReduce has 'sharded: true', the output collection cannot be an existing unsharded collection. We fail the mapReduce command in this case.

Scope of changes:

https://docs.mongodb.com/manual/reference/command/mapReduce/index.html – add a note to the section that describes the "sharded" parameter in the out stage/

https://docs.mongodb.com/manual/reference/method/db.collection.mapReduce/index.html – ditto

clarify: core/map-reduce-sharded-collections.html

Impact to other docs outside of this product:

MVP:

Resources:

Engineering Ticket Description:

mapReduce with a sharded output collection assigns the UUID obtained from the config server to the final output collection. mapReduce "replace" will drop the existing output collection, which has the same UUID as the new output collection. Two-phase-drop may cause the dropCollection to finish after the renameCollection finishes, erroneously removing the UUIDCatalog entry for the output collection.