That documentation you linked to put you on the right track! If you click on the %SYS.Maint.Bitmap link it will take you to the class reference which contains some examples of how to run the bitmap cleanup:
d ##class(%SYS.Maint.Bitmap).Namespace("Samples",1,1,"2014-01-17 09:00:00")
This utility was only added to the product around version 2015, I believe. It should run automatically as part of an Ensemble message purge. If you need to perform this maintenance on an older version, rebuilding the index can be done though this is a more involved process, especially on a live system.
If needed, you can reach out to the WRC who can provide a utility that performs a similar function to %SYS.Maint.Bitmap for older versions.
Hope that helps!