add example of a trait with a where clause

Author: tbrowder

doc/Type/Signature.pod6

@@ -507,6 +507,10 @@ C<Array> full of C<Scalar>s as described above, C<is raw> will instead cause
 the parameter to produce a C<List>.  Each element of that list will be bound
 directly as raw parameter.
 
+Traits can be followed by the where clause:
+
+    sub ip-expand-ipv6($ip is copy where m:i/^<[a..f\d\:]>**3..39$/) {...}
+
 =head1 Methods
 
 =head2 method params