Modified the doc string of value_counts for DataFrame and series when sort arg is set to false
 [ ✓] closes #51420 

Added the explanation of 'sort' keyword for pandas.DataFrame.value_counts and pandas.Series.value_counts. When 'sort' is set to False, _value_counts_ method sorts the _dataframe_ by index. For _series,_ it preserves the order of the data.

The doctest fails due to missing explanation of the return value in value_counts. As it is not related to this issue, it should be handled separately.

Thx @VomV 