I  think following the docstring styles from PEP-257 makes docstrings clearer, as it forces you to say *what the function does* which is much more direct and concrete, instead of *what the function is for* which can be less clear.

In this case, it would be something like:

    '''Return the given ``activity_list`` with activities from certain users removed.

    The users whose activities will be filtered are defined to the
     :ref:`ckan.hide_activity_from_users` config setting.

    A *new* filtered list is returned, the given ``activity_list`` itself is not modified.

    '''

That also adds a bit more info like about how the config option is used.

Mentioning that the original list isn't changed might be too verbose, up to you

I think it also helps that if you can't describe what the function does in a first-line like that that fits in 80 columns, that might be a sign that your function needs breaking into smaller ones