bug#48248: tr docs: mention what to expect now vs. future

[積丹尼 Dan Jacobson]

(info "(coreutils) tr invocation") says
   Currently ‘tr’ fully supports only single-byte characters.
   Eventually it will support multibyte characters; when it does, the ‘-C’

And the man page has no warning at all. #BUG. Please add one.

BTW,

$ echo 哇|tr 哇 123
123

It works fine! (People will think.)

Therefore be sure to mention just when it will start failing, with real 
examples,

$ echo 哇|tr 哇 1
111
$ echo 哇|tr 哇 1234
123
$ echo 哇哇|tr 哇 1234
123123
$ echo 哇哇姑五湖|tr 哇 1234
1231231__五湖

$ echo "22°56'52.2\"N 121°14'09.3\"E" | tr ° d
22dd56'52.2"N 121dd14'09.3"E

Also on the INFO page, give examples of, for the same input,
what to expect now, and what to expect in the future.