Pie Chart: Clearer UX when limit is applied

There is a default limit of 25 rows in a Pie Chart, presumably to prevent situations where we render thousands of pie slices and crash the browser for queries which return that much data. However, the limit is currently a bit hard to - the limit is applied when there's no limit set, since it's a default. Currently, that is indicated in the UI via the placeholder for the limit field, but there could be a description. and also, the docs do not mention this behavior. - when _viewing_ a Pie Chart, it's not clear that a limit has been applied. A user who didn't configure the dashboard wouldn't necessarily know about this limitation or that any data is being filtered out when they view a Pie Chart. Changes I would recommend making: - add a description to the limit field - update the docs to mention this behavior, including recommendation to sort the data via query or transform to surface the data they care about - add a hint in the visualization itself if the limit is less than the total number of rows (like "showing 25 of 100 rows")