One clarifier: the documentation quoted above is regarding custom fields rather than custom expressions (or metrics). Eva's screenshots are from the custom field UI, and Raz's are from the custom expression UI. I occasionally get momentarily confused about the distinction myself.
Custom fields basically let you do simple arithmetic on fields or between fields in order to create a new field, and can't include aggregations because that would "squash" the field. Custom expressions/metrics can include arithmetic and basic aggregation functions (sum, average, etc.), but those aggregation functions can't include additional aggregations inside of themselves.