DOC:Updated as suggested in the comments of PR#20211
- [x] PR title is "DOC: update the docstring"
- [x] closes #20211 
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

The following is added for the doc string :
         """
	Return a boolean if this key is in the index.

         Parameters
         ----------
         key : object
         key to be searched.

         Returns
         -------
         boolean
         result of the search.

         See Also
         --------
         Index.isin

         Examples
         --------
         >>> pd.CategoricalIndex([2000, 2001, 2012]).contains(2001)
         True
         >>> pd.CategoricalIndex([2000, 2001, 2012]).contains(2222)
         False
         """

The suggestions mentioned in the pull request#20211 were followed. Probably this can help close pull request#20211.