Merge pull request #2677 from perl6/clarify-gcd-zero

Clarify gcd with zero results

Author: cfa

doc/Language/operators.pod6

@@ -1164,6 +1164,7 @@ Please note that this has not yet been implemented.
     multi sub infix:<gcd>($a, $b --> Int:D)
 
 Coerces both arguments to L<Int|/type/Int> and returns the greatest common divisor.X<|greatest common divisor operator>
+If one of its arguments is 0, the other is returned (when both arguments are 0, the operator returns 0).
 
 =head2 infix C«lcm»