grep: document how to specify non printing characters

[Dan Jacobson]

The challenge: grep for all non-ascii bytes.
$ LC_ALL=C grep '[\x80-\xFF]' file.
Well, firstly, the grep man page doesn't say how one represents
'[\x80-\xFF]' to grep. No word on if it is \x \0x 0x, or "sorry, no
way, so don't think about it", or even if one must just use the raw
character (works?). Of course there is no [^[:ascii:]].