bug#49193: Document require-final-newline vs. readonly files

[Eli Zaretskii]

> From: 積丹尼 Dan Jacobson
>  <jidanni@jidanni.org>
> Date: Wed, 23 Jun 2021 23:14:56 +0800
> 
> (info "(emacs) Customize Save") says:
>    If the value of the variable ‘require-final-newline’ is ‘t’, saving or
>    writing a file silently puts a newline at the end if there isn’t already
>    one there.  If the value is ‘visit’, Emacs adds a newline at the end of
>    any file that doesn’t have one, just after it visits the file...
> 
> OK, but mention if this even affects readonly files / owned by other users
> that we visit.

There's nothing to mention, because this feature doesn't change how
saving to an unwritable file behaves in Emacs.  The feature works on
the buffer text, not on the buffer's file.