Added 2 new fields to the refineShardKey command:
enforceUniquenessCheck: Controls whether this command verifies that any unique indexes are prefixed by the new shard key pattern if unique is true. If true then it will verify and if false then it won't. Defaults to true.

Not strictly required to be in 6.0 docs, but eventually in the docs. These new fields are mostly for use by cluster to cluster replication and documentation is mostly for other mongo engineers. Not sure if there's a better place to put it aside from the docs.

Description of Linked Ticket

(Analogous to SERVER-63732 but for the refineCollectionShardKey command. An implicitlyCreateIndex parameter isn't relevant for refineCollectionShardKey because the command won't attempt to create an index even when the collection is empty.)

enforceUniqueness defaults to true and when set to false, ignores uniqueness property when validating indexes for refineCollectionShardKey.

Regardless of both settings, refineCollectionShardKey should still return an error if there are no index that is compatible with the new shard key pattern.