Added docs for Bool.Numeric

doc/Type/Bool.pod6

@@ -71,6 +71,15 @@ C<$count> an infinite L<Seq|/type/Seq> of C<Bool>s is returned.
     say Bool.roll(3);                                 # (True False False)
     say Bool.roll(*);                                 # (...)
 
+=head2 routine Numeric
+
+    multi method Numeric(Bool:D) returns Int:D
+
+Returns the value part of the C<enum> pair.
+
+    say False.Numeric;                                # 0
+    say True.Numeric;                                 # 1
+
 =head1 Operators
 
 =head2 prefix ?