bug#30643: Mention . won't match illegal characters

[積丹尼 Dan Jacobson]

$ man grep says

       The period . matches any single character.

OK but it should also mention that illegal characters for the current
locale will never be matched by ".".

$ LC_ALL=C           grep  .東看 x.html|qprint -e
          <p>=E5?=91=E6=9D=B1=E7=9C=8B</p>
$ LC_ALL=zh_TW.UTF-8 grep  .東看 x.html|qprint -e
$

Also mention it on (info "(grep) Fundamental Structure").