Update Nil.pod6

At first glance, it seems that Nil can be assigned to any container. Extra explanation for hash assignment.

doc/Type/Nil.pod6

@@ -53,7 +53,15 @@ C<Nil>.
 X<|Nil assignment>
 When assigned to a L<container|/language/containers>, the C<Nil> value (but not any subclass of
 C<Nil>) will attempt to revert the container to its default value; if no
-such default is declared, Perl 6 assumes C<Any>. However, if the container
+such default is declared, Perl 6 assumes C<Any>. 
+
+Since a hash assignment expects two elements, use C<Empty> not C<Nil>, eg.
+
+    my %h = 'a'..'b' Z=> 1..*;
+    # stuff happens
+    %h = Empty; # %h = Nil will generate an error
+
+However, if the container
 type is constrained with C<:D>, assigning C<Nil> to it will immediately throw
 an exception.  (In contrast, an instantiated C<Failure> matches C<:D>
 because it's a definite value, but will fail to match the actual nominal