>> +(defun vc-root-version-diff (_files rev1 rev2)
>> +  "Report diffs between revisions of the whole tree in the repository history."
>
> This doc string doesn't describe the arguments, and doesn't tell how
> those arguments are populated in interactive invocations.

I copied the doc string from `vc-version-diff' that doesn't describe its
arguments.  Now I fixed the doc string of `vc-version-diff' as well below.

> I also wonder what would this mean for a VCS such as CVS or RCS, where
> there's no meaning of a "tree revision".  I'm guessing this is why the
> old code deduced files to diff.  See also the description of vc-diff
> in the Emacs manual.

`vc-root-diff' is not applicable to VCS without tree revisions, and
the manual says nothing about this.  Maybe the manual should say this
explicitly for `vc-root-diff' as it says it for `vc-print-root-log'
in (info "(emacs) VC Change Log")

     ‘C-x v L’ (‘vc-print-root-log’) displays a ‘*vc-change-log*’ buffer
  showing the history of the entire version-controlled directory tree
  (RCS, SCCS, CVS, and SRC do not support this feature).

> Finally, we need documentation changes to go with this change of
> user-visible behavior.

This feature is already documented in (info "(emacs) Old Revisions")

  ‘C-x v D’
       Compare the entire working tree to the revision you started from
       (‘vc-root-diff’).  With a prefix argument, prompt for two revisions
       and compare their trees.

It didn't work as documented before properly implemented by the patch.