Triage notes: it looks like the only doc change is here: https://docs.mongodb.com/manual/core/sharding-balancer-administration/index.html#migration-thresholds.

Also, "migration thresholds" links to itself on this page. Does it make sense to explain this further?

Document Migration Thresholds:

• In MongoDB 3.2 or earlier, the migration thresholds are calculated by difference in number of chunks between the shard with the most chunks for the collection and the shard with the fewest chunks for that collection.
• However, the migration thresholds have been changed since MongoDB 3.4 in SERVER-24721, and also 4.0 in SERVER-35003. We'd need to update the document on the migration thresholds for MongoDB 3.4 or above.