bug#23451: 25.0.93; Clarify the dependency on find/grep for platforms not having those tools

[Eli Zaretskii]

> From: npostavs@users.sourceforge.net
> Date: Mon, 29 May 2017 08:43:58 -0400
> Cc: 23451@debbugs.gnu.org, Kaushal Modi <kaushal.modi@gmail.com>
> 
> This seems to work:
> 
>     find ... -exec sh -c 'grep -i -E -nH -e turn-on-eldoc-mode "$@" ; [ $? 
> -le 1 ]' sh '{}' +

Who said sh is available?

I think we should simply test explicitly for 'find' and 'grep' being
available, before we run the command.  Otherwise, it sounds like we
will increase the complexity and the fragility of the feature, to very
little gain.