Fixed and clarified docstrings for client.py

[kanari1]

There were some confusing semantics and errors in the docstrings for the functions in spotipy/client.py files, so I clarified and fixed some of the errors in those docstrings.

• Fixed docstring for current_user_following_users function to correctly state that it checks if the current user is following certain users, not certain artists
• Expanded docstring for playlist_change_details function to also include that changes can be made to the collaborative state and description
• Made semantical edits to clarify certain docstrings