bug#23904: Btrfs clone support in copy operations

[Eli Zaretskii]

> Cc: dmantipov@yandex.ru, 23904@debbugs.gnu.org, sbaugh@catern.com,
>  kieran@kcolford.com
> From: Paul Eggert <eggert@cs.ucla.edu>
> Date: Sun, 11 Sep 2016 15:44:45 -0700
> 
> Come to think of it, the user manual already documents 
> write-region-inhibit-fsync, so it was a bit odd that the reference manual was 
> silent about it. I installed the attached further patch to document 
> write-region-inhibit-fsync in the reference manual, and to help the reference 
> manual avoid a dependency on the coreutils manual.

Thanks, I think now that subsection makes more sense.

But please remove the explicit @vindex entry, since @defvar already
creates one, and in the correct place.