Docs adjustments and athena iceberg test fix [ci skip]

Author: anuunchin

docs/website/docs/dlt-ecosystem/destinations/iceberg.md

@@ -120,7 +120,7 @@ The [S3-compatible](./filesystem.md#using-s3-compatible-storage) interface for G
 The `az` [scheme](./filesystem.md#supported-schemes) is not supported when using the `iceberg` table format. Please use the `abfss` scheme. This is because `pyiceberg`, which dlt used under the hood, currently does not support `az`.
 
 ## Table format `merge` support (**experimental**)
-The [`upsert`](../../general-usage/merge-loading.md#upsert-strategy) merge strategy is supported for `iceberg`.
+The [`upsert`](../../general-usage/merge-loading.md#upsert-strategy) merge strategy is supported for `iceberg`. This strategy requires that the input data contains no duplicate rows based on the key columns, and that the target table also does not contain duplicates on those keys.
 
 :::caution
 The `upsert` merge strategy for the filesystem destination with Iceberg table format is **experimental**.

tests/load/pipeline/test_merge_disposition.py

@@ -440,7 +440,10 @@ def r(data):
     table_data = load_tables_to_dicts(p, "parent", "parent__child", exclude_system_cols=True)
     if merge_strategy == "upsert":
         # merge keys will not apply and parent will not be deleted
-        if destination_config.table_format in ["delta", "iceberg"]:
+        if (
+            destination_config.table_format in ["delta", "iceberg"]
+            and destination_config.destination_type != "athena"
+        ):
             # delta merges cannot delete from nested tables
             assert table_counts == {
                 "parent": 3,  # id == 3 not deleted (not present in the data)