bug#35019: 27.0.50; Incomplete docstring for vc-version-diff and vc-version-ediff

[Eli Zaretskii]

> From: Philipp Stephani <p.stephani2@gmail.com>
> Date: Wed, 27 Mar 2019 13:15:07 +0100
> 
> The docstrings of the commands vc-version-diff and vc-version-ediff
> don't describe what their arguments mean.  They also don't describe the
> special cases for revisions that are nil or the empty string.

Thanks, fixed on the emacs-26 branch.

(I didn't see the need to document the empty string case, it looks
like an internal implementation detail to me.)