[doc] Perlop: mention best way to get a real 0

[jidanni]

Perlop says

   Relational Operators
       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.

Perhaps add

If you want a real 0, then add 0 to it.
   print 3 == 3; # 1
   print 3 == 4; # nothing
   print 0 + ( 3 == 4 ); # 0