Improve docs for when Darker can't overwrite files

[akaihola]

In #778, @haohu321 wondered why --revision=HEAD^..HEAD won't overwrite reformatted Python source files.

I answered:

This is intentional. It only makes sense to modify files on disk when comparing old revisions to the working tree (--revision=<old>..:WORKTREE:, shorthand --revision=<old>).

If you're comparing already committed revisions (--revision=<older>..<old>), and some lines were modified and not properly formatted between those revisions, it's not safe to reformat the files on disk in the working tree since there may have been later modifications either in commits after <old> or in the working tree.

So it's important to be aware that HEAD is different from :WORKTREE:.

Let's improve the documentation and/or error messages to make this clear.

Originally posted by @akaihola in #778 (comment)