Improve docs for add file procedures in Iceberg

Reword some aspects to make it more precise and ensure
that it is clear that the catalog and system schema must
be specified.

docs/src/main/sphinx/connector/iceberg.md

@@ -568,11 +568,13 @@ nested directories, or `false` to ignore them.
 (iceberg-add-files)=
 #### Add files
 
-The connector can add files from tables or locations if
+The connector can add files from tables or locations to an existing table if
 `iceberg.add_files-procedure.enabled` is set to `true` for the catalog.
 
-Use the procedure `system.add_files_from_table` to add existing files from the Hive 
-table or `system.add_files` to add existing files from specified locations. 
+Use the procedure `system.add_files_from_table` to add existing files from a
+Hive table or `system.add_files` to add existing files from a specified location
+to an existing table.
+
 The data files must be the Parquet, ORC, or Avro file format.
 
 :::{warning}
@@ -584,17 +586,19 @@ relevant schema and table names supplied with the required parameters
 `schema_name` and `table_name`:
 
 ```sql
-ALTER TABLE testdb.iceberg_customer_orders EXECUTE add_files_from_table(
+ALTER TABLE testdb.iceberg_customer_orders 
+EXECUTE example.system.add_files_from_table(
     schema_name => 'testdb',
     table_name => 'hive_customer_orders')
 ```
 
-You need to provide a `partition_filter` argument to add files from specified partitions.
+Use a `partition_filter` argument to add files from specified partitions.
 The following example adds files from a partition where the `region` is `ASIA` and
 `country` is `JAPAN`:
 
 ```sql
-ALTER TABLE testdb.iceberg_customer_orders EXECUTE add_files_from_table(
+ALTER TABLE testdb.iceberg_customer_orders 
+EXECUTE example.system.add_files_from_table(
     schema_name => 'testdb',
     table_name => 'hive_customer_orders',
     partition_filter => map(ARRAY['region', 'country'], ARRAY['ASIA', 'JAPAN']))
@@ -604,7 +608,8 @@ In addition, you can provide a `recursive_directory` argument to migrate a
 Hive table that contains subdirectories:
 
 ```sql
-ALTER TABLE testdb.iceberg_customer_orders EXECUTE add_files_from_table(
+ALTER TABLE testdb.iceberg_customer_orders 
+EXECUTE example.system.add_files_from_table(
     schema_name => 'testdb',
     table_name => 'hive_customer_orders',
     recursive_directory => 'true')
@@ -614,12 +619,13 @@ The default value of `recursive_directory` is `fail`, which causes the procedure
 to throw an exception if subdirectories are found. Set the value to `true` to add 
 files from nested directories, or `false` to ignore them.
 
-`add_files` procedure supports adding files from a specified location.
+The `add_files` procedure supports adding files from a specified location.
 The procedure does not validate file schemas for compatibility with
 the target Iceberg table. The `location` property is supported for partitioned tables.
 
 ```sql
-ALTER TABLE testdb.iceberg_customer_orders EXECUTE add_files(
+ALTER TABLE testdb.iceberg_customer_orders 
+EXECUTE example.system.add_files(
     location => 's3://my-bucket/a/path',
     format => 'ORC')
 ```