Update this page: https://mms.mongodb.com/help-hosted/v1.4/core/administration-interface/#grooms

With more information about grooms.

Suggested text:
MMS OnPrem 1.4 and later performs periodic garbage collection that removes unused blocks from the Blockstore DB and frees up space. This process is called a Groom.
A scheduling process will determine when space can be saved for a particular replica set being backed up and create a Groom job to be completed. The groom process will force all new data to be written to a new location, copy all existing, needed data from the old location to the new one, update references to maintain data relationships, and then drops the old database. As a result of a groom, you may notice that blockstore filesize will fluctuate, sometimes dramatically.