sdks/python/apache_beam/io: add note on output_fn ctor arg to indic…

…ate that It is unused and only retained for backward compatibility (#32635)

In this commit, we add unused note on `output_fn` constructor
argument in `WriteToFiles` class and only retained for backward
compatibility.

Signed-off-by: Mohamed Awnallah <mohamedmohey2352@gmail.com>

sdks/python/apache_beam/io/fileio.py

@@ -557,6 +557,8 @@ def __init__(
         class signature or an instance of FileSink to this parameter. If none is
         provided, a ``TextSink`` is used.
       shards (int): The number of shards per destination and trigger firing.
+      output_fn (callable, optional): A callable to process the output. This
+        parameter is currently unused and retained for backward compatibility.
       max_writers_per_bundle (int): The number of writers that can be open
         concurrently in a single worker that's processing one bundle.
     """