Since the balancer's policy drastically changes in v6.1.0, it must be documented in the public release notes.

Notable changes:

• The assumption that all chunks contain the same amount of data is not true anymore
• The balancer is evenly distributes data rather than chunks (related to DOCS-15340): a user-visible consequence is that manually creating chunks does not necessarily mean the balancer will move them around
• The auto-splitter does not exist anymore: chunks are created by the balancer only when it is needed to move data (related to DOCS-15397)

This is just a placeholder to remember that the changes need to be documented, I guess garaudy.etienne@mongodb.com can help the DOCS team setup a proper announcement.