bug#28615: [PATCH] Clarify what grep-read-files wants

[Eli Zaretskii]

> From: Allen Li <vianchielfaura@gmail.com>
> Date: Mon, 9 Oct 2017 21:43:59 -0700
> Cc: 28615-done@debbugs.gnu.org
> 
> Why did you not change the collection function?

Because there could be a file called literally like one of the
shorthands in grep-files-aliases, or whose name includes those
shorthands as a substring.

> As I noted in the bug, most of the completions that would be
> provided by read-file-name-internal don't work

IMO, that's okay, because wildcards cannot be meaningfully completed
on.