feat(pivot-table): add result aggregation to tabular exports

docs/docs/using-superset/pivot-result-exports.md

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+
+# Pivot result aggregation exports
+
+Pivot Table result aggregation summarizes the grouped metric values returned by
+the chart query. For example, with grouped values 1, 9, 20 and 100, a saved
+**Median** aggregation produces a grand summary of 14.5. It does not calculate the median
+of intermediate subtotals. Query filters and row limits apply before this step.
+
+## Saved aggregation settings
+
+The chart control panel exposes **Show values as**, but does not expose a
+result-aggregation selector. This export behavior applies to charts whose saved
+configuration already contains an `aggregateFunction`, for example from a
+version with an aggregation selector. This page does not describe a new control
+available in the chart editor.
+
+A missing, unrecognized or **Metric** setting keeps metric-definition behavior.
+Saved settings such as **Sum**, **Average** and **Median** apply a second
+aggregation to the returned grouped results.
+
+Row and column subtotals can be enabled independently of grand totals. A shared
+total combining different metrics is blank because those metrics do not have a
+single meaningful aggregate. This also applies when metrics are placed on rows
+or the table is transposed.
+
+## Fractions and formatting
+
+**Sum as Fraction of Total**, **Rows** or **Columns**, and the corresponding
+**Count** choices, use only their own metric in the denominator. Result
+aggregation takes precedence over the separate **Show values as** setting.
+Without a saved result aggregation, that setting applies to database-computed
+metric values.
+
+Formatted results display fractions as percentages, including when a metric has
+a currency override. CSV exports retain raw numeric ratios. Excel fractions
+remain numeric cells with a percentage number format, so they can be used in
+formulas. Other formatted results retain their configured formats; AUTO currency formatting uses the currencies of the
+records contributing to each cell or subtotal.
+
+## Missing values
+
+Numeric result reducers ignore NULL observations. **First** takes the first
+non-NULL value, while **Last** retains the final value even when it is NULL.
+Sample variance and sample standard deviation are zero for a single numeric
+observation. Stored database totals are not additional observations: result
+aggregation uses the leaf query results.