bug#40948: 26.3; Docstring of substitute-env-vars

[Eli Zaretskii]

> From: Marcus Harnisch <marcus.harnisch@gmx.net>
> Date: Tue, 28 Apr 2020 22:22:52 +0200
> 
> With commit ee041f2d07b6ed485dc34c115588f973f046c9d4, changes to the
> parameters of `substitute-env-vars' were introduced and its docstring
> changed. While the optional parameter is named WHEN-UNDEFINED, the
> docstring says WHEN-DEFINED. It would appear that the dosctring is
> right, but I'll leave that decision to the experts.

I modified the doc string to match the argument name.  (I'm not sure
how to judge which one is correct, because IMO neither WHEN-DEFINED
nor WHEN-UNDEFINED is correct.  I would call that argument
HANDLE-UNDEFINED or something to that effect.)

Also note that there's an inconsistency in how the value of this
variable is used: if it's a function, then the return value of nil
means leave the variable reference unchanged, but if it is not a
function, the value of nil means to replace the reference with an
empty string, and a non-nil value means leave it unchanged.