>> +(defcustom multi-file-diff-unsaved 'save-buffers
>> +  "What to do with unsaved edits when showing multi-file replacements as diffs.
>> +If the value is `save-buffers', save unsaved buffers before creating diff.
>> +If the value is `use-file', use text from the file even when the visiting
>> +file buffer is modified.
>> +If the value is `use-modified-buffer', use text from the file-visiting
>> +modified buffer to be able to use unsaved changes.  However, when the file
>> +is not visited in a buffer, or the buffer is not modified, still read
>> +contents from the file."
>
> Please use consistent wording to describe the same entities.  If you
> use "file-visiting buffer", use it everywhere, when you sometimes use
> that and sometimes "visiting file buffer", you are already half way to
> confusing the reader.

Ok, fixed below.

> Also, this part:
>
>> +                                                    However, when the file
>> +is not visited in a buffer, or the buffer is not modified, still read
>> +contents from the file."
>
> Seems to describe an implementation detail, and I don't think it
> should be there.  E.g., what if the file visited by the buffer no
> longer exists?

If the file visited by the buffer no longer exists, then
the standard error is signaled.