docs: note that apache#19137 chose append; replace is the original behavior

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: adriangb

datafusion/datasource/src/table_schema.rs

@@ -150,6 +150,13 @@ impl TableSchema {
         // isolation is automatic). The previous `Arc::get_mut().expect()`
         // appended in place and panicked whenever the `Arc` was shared, since
         // owning `self` does not imply sole ownership of the inner `Arc`.
+        //
+        // Note: this setter originally replaced the partition columns. #19137
+        // switched it to append (when introducing the inner `Arc`), but that
+        // was just picking *a* behavior for repeated calls — appending is not
+        // required or more correct than replacing. Replacing matches the usual
+        // `with_*` builder convention and avoids accidentally duplicating
+        // columns, so we restore it here.
         self.table_partition_cols = Arc::new(partition_cols);
         let mut builder = SchemaBuilder::from(self.file_schema.as_ref());
         builder.extend(self.table_partition_cols.iter().cloned());