Document Rational.norm as deprecated

Author: zoffixznet

doc/Type/Rational.pod6

@@ -109,6 +109,9 @@ Returns a list of the numerator and denominator.
 
     method norm(Rational:D: --> Rational:D)
 
+B<DEPRECATED as of 6.d>. The method is no longer needed, because as of 6.d
+language version, it's required for C<Rational> type to be normalized on creation.
+
 Returns a normalized Rational object, i.e. with positive denominator, and
 numerator and denominator coprime.