docs: document "Expand subtotals by default" table option

references/chart-types/table.mdx

@@ -195,6 +195,17 @@ You can add subtotals to your tables by selecting `Show subtotals` in the chart
 
 To use subtotals, you need to have at least 2 or more dimensions in your table visualization.
 
+### Expand subtotals by default
+
+When `Show subtotals` is enabled, you can also enable `Expand subtotals by default` to control how subtotal groups render when the chart first loads:
+
+- **Off** (default): subtotal groups load collapsed. Viewers click each group to expand it and see the underlying rows.
+- **On**: subtotal groups load expanded, showing every row beneath each subtotal.
+
+This setting is saved with the chart, so anyone viewing the saved chart or dashboard tile sees the state you chose. Viewers can still expand or collapse individual groups while exploring.
+
+The option is unavailable when subtotals are off or when `Show metrics as rows` is enabled on a pivot table.
+
 ## Freeze columns
 
 If you have a wide table, you may want some columns to be locked to the left while you're scrolling. Click on the lock icon beside the column(s) you want to keep pinned to the left of your table visualization to lock them in place.