bug#16744: 24.3.50; REGRESSION: C-h v does not show doc for `ediff-highlight-all-diffs'

[Drew Adams]

> That's because of the ediff-defvar-local macro:
> 
>   (defcustom ediff-highlight-all-diffs t
>     "If nil, only the selected differences are highlighted.
>   Otherwise, all difference regions are highlighted, but the
> selected region is
>   shown in brighter colors."
>     :type 'boolean
>     :group 'ediff-highlighting)
>   (ediff-defvar-local ediff-highlight-all-diffs t "")
> 
> It is invoked (here and elsewhere) with an empty string as the doc,
> so that's what you get.

I see.  Seems like such a macro, if really needed, should at least
reuse the main doc string, in _addition_ to adding any local-binding
doc string.  Or perhaps allow for either adding or replacing.
But it should not require duplicating the literal string as an
argument, just to provide doc for the local var.