bug#29262: 26.0; doc string of `find-grep-dired'

[Eli Zaretskii]

> Date: Sat, 11 Nov 2017 13:32:37 -0800 (PST)
> From: Drew Adams <drew.adams@oracle.com>
> 
> The doc string is not as clear as it could be.  It says:
> 
> Find files in DIR matching a regexp REGEXP and start Dired on output.
>      ^^^^^        ^^^^^^^^^^^^^^^^^
> 
> What does it mean for a file to match a regexp?  This could easily be
> misunderstood as the file name matching the regexp.
> 
> Please consider making the text clearer, e.g., along the lines of what
> is said in the Emacs manual.  It speaks of files "that contain a match
>                                                        ^^^^^^^
> for REGEXP."

Thanks, fixed.