docs(batch-exports): document backfill estimation and progress tracking

contents/docs/cdp/batch-exports/index.mdx

@@ -131,9 +131,11 @@ From here, you can create a new backfill by clicking the "Start backfill" button
 
 ![create historic export](https://res.cloudinary.com/dmukukwp6/image/upload/2024_08_09_at_14_54_54_90338968ac.png)
 
-This will immediately schedule a series of batch export runs that fall within the bounds selected, based on the batch export's frequency.
+After you start a backfill, PostHog first estimates the total number of records to export and validates the date range.
 
-The start date of a backfill may be adjusted depending on when the earliest available data is. For example, if you choose a start date of 2024-01-01 but the earliest event captured in PostHog is 2024-01-02, the start date will be adjusted to 2024-01-02. On the other hand, the end date of the backfill cannot be set past the current time, as our engineers have yet to figure out time traveling to allow us to export events in the future.
+The start date of a backfill may be adjusted depending on when the earliest available data is. For example, if you choose a start date of 2026-01-01 but the earliest event captured in PostHog is 2026-01-02, the start date is adjusted to 2026-01-02. On the other hand, the end date of the backfill cannot be set past the current time, as our engineers have yet to figure out time traveling to allow us to export events in the future.
+
+If the estimation finds no matching data to export, the backfill completes immediately rather than creating empty batch runs.
 
 
 ## FAQ