[Documentation] `RefreshIndicator`:  Provide more documentation for `ScrollNotificationPredicate`

https://github.com/flutter/flutter/blob/b8c3662a37a2b39ba0308f2c1be6e8e94f0867ec/packages/flutter/lib/src/material/refresh_indicator.dart#L183

Please provide more description as to what it does and how to use it.
I don't know how to use it after I read the documentation and inspected the repo code.