This text is also present in the 5.005_03 documentation.

Even the documentation that came with perl5.004 includes those two paragraphs:

    m     Treat string as multiple lines.  That is, change "^"
          and "$" from matching at only the very start or end of
          the string to the start or end of any line anywhere
          within the string,

    s     Treat string as single line.  That is, change "." to
          match any character whatsoever, even a newline, which
          it normally would not match.
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Although it lacks the followup explanation of the interactions between /s, /m, and $*.

The original poster must have a very old version of Perl. Complaining about a lack of documentation in such an old version of Perl is pointless. The problem was found and fixed years ago; the solution is to upgrade.