[perl #33765] perlop: mention why 'print !!0' doesn't

This was done as part of 1ca345ed. perlop now contains:

       Perl operators that return true or false generally return values
       that can be safely used as numbers.  For example, the relational
       operators in this section and the equality operators in the next
       one return 1 for true and a special version of the defined empty
       string, "", which counts as a zero but is exempt from warnings
       about improper numeric conversions, just as "0 but true" is.

-doy

• [perl #33765] perlop: mention why 'print !!0' doesn't by Jesse Luehrs via RT