Obviously there is a lot of overlap between the grid/core with the gantt.
But for example lets say I want to lookup what
intervals: [ recurrentstartdate "on sat at 0:00" ]
means, is there any other doc to search?
Try the docs, and use the search field:
https://bryntum.com/docs/gantt/#SchedulerPro/model/CalendarIntervalModel#field-recurrentStartDate