bug#81591: 31.0.91; getenv fallback to selected frame is inaccurately documented

> From: "Aaron L. Zeng" <[email protected]>
> Date: Sun, 09 Aug 2026 15:19:15 -0400
> 
> Hello,
> 
> I noticed a discrepancy in the description of how `getenv' (with a nil
> FRAME argument) behaves when a variable is not found in
> process-environment but is found in the selected frame's environment
> parameter.  getenv's docstring says:
> 
>     If optional parameter FRAME is non-nil, then it should be a
>     frame.  This function will look up VARIABLE in its ‘environment’
>     parameter.
> 
>     Otherwise, this function searches ‘process-environment’ for
>     VARIABLE.  If it is not found there, then it continues the search
>     in the environment list of the selected frame.
> 
> So, when FRAME is nil, I would expect that any variables not present
> in `process-environment' should be looked up in the selected frame's
> environment.  However, this does not happen for SSH_CONNECTION
> (reading the source of getenv_internal, it seems to actually be a
> special case for DISPLAY and no other variables).  I noticed I had to
> explicitly pass the selected frame to the `getenv' function.
> 
> Looking at the history of getenv_internal, it seems quite deliberate
> that DISPLAY is a special case and that no other variables were
> intended to be looked up in the selected frame implicitly.  If that's
> the case, I would suggest the docstring should be updated to mention
> it.

Thanks, should be fixed now.

I took the opportunity to update the ELisp manual as well, which was
also outdated and missing details in this regard.