[From emacs-devel]

>> +(defcustom Buffer-menu-group-by nil
>> +  "If non-nil, buffers are grouped by function."
>> +  :type 'function
>> +  :group 'Buffer-menu
>> +  :version "30.1")
>
> Please consider letting users choose a symbol, not a function.  Each
> symbol can be mapped to a function, but having a user option whose
> values are functions makes it harder for users to customize the
> option.

Ok, now the type is changed to 'symbol'.  Here is the complete patch: