bug#27785: tr: mention that leading zeros are a no-no in e.g., \0377

[積丹尼 Dan Jacobson]

In (info "(coreutils) Character sets")

     ‘\OOO’
          The 8-bit character with the value given by OOO, which is 1 to
          3 octal digits.  Note that ‘\400’ is interpreted as the
          two-byte sequence, ‘\040’ ‘0’.

It is very important to also add here:

          and \0377 is interpreted as \037 and 7.

(You add the quotes. I can't with my keyboard.)