Improve docstring for PGBKCVOperation (#37946)

* Improve docstring for PGBKCVOperation

* Fix docstring is satisfy lint

* Add costring for PGBKCVOperation

* Revert untended .gitignore changes

Author: Subramanya-Veeregowda

sdks/python/apache_beam/examples/snippets/transforms/aggregation/groupby_test.py

@@ -39,7 +39,6 @@
 from .groupby_two_exprs import groupby_two_exprs
 
 
-
 class UnorderedList(object):
   def __init__(self, contents):
     self._contents = list(contents)

sdks/python/apache_beam/runners/worker/operations.py

@@ -1234,6 +1234,11 @@ def flush(self, target):
 
 
 class PGBKCVOperation(Operation):
+  """Partial group-by-key operation.
+
+  This operation handles grouped values with
+a combine function applied.
+  """
   def __init__(
       self, name_context, spec, counter_factory, state_sampler, windowing=None):
     super(PGBKCVOperation,