bug#38675: 27.0.50; define-obsolete-face-alias definition does not match documentation

[Eli Zaretskii]

> From: James Felix Black <jfb@homonculus.net>
> Date: Thu, 19 Dec 2019 12:49:01 -0500
> 
> This was discovered in `haskell-mode` [1]. The documentation of  
> define-obsolete-face-alias claims that `when` is optional, but the macro 
> definition does not follow the docs. This patch fixes the problem.

I'm not sure this isn't a confusion caused by ambiguous wording of the
doc string.  I think by "if provided" it means that WHEN can be nil.
The ELisp manual, btw, doesn't have this problem.

Would it be okay to modify the doc string instead to say WHEN is
required, but can be nil?

Thanks.