>> +(defcustom multi-file-diff-unsaved 'use-file
>> +  "A choice defining what to do with unsaved changes.
>
> This first sentence is too general.  I suggest
>
>   What to do with unsaved edits when showing multi-file replacements as diffs.

Fixed, and changed the default to 'save-buffers' like Eshel suggested.

>> +(defun multi-file-replace-regexp-as-diff (files regexp to-string &optional delimited)
>> +  "Show replacements in FILES matching REGEXP with TO-STRING as diff.
>
>   Show as diffs replacements of REGEXP with TO-STRING in FILES.

Fixed with more small changes: