Added docs for method keyof in class Hash. lizmat++, jnthn++

doc/Type/Hash.pod

@@ -340,6 +340,28 @@ C<(Any)>.
     say %h2.default;                                       # 1
     say %h2{'apples'} + %h2{'bananas'};                    # 4
 
+=head2 method keyof
+
+Defined as:
+
+    method keyof 
+
+Usage:
+
+    Hash.keyof
+
+Returns the type constraint for the keys of the invocant. For 
+normal hashes the method returns the coercion type C<(Str(Any))>
+while for L<object hashes|/type/Hash#Object_hashes_and_type_constraints>
+the type used in the declaration of the C<Hash> is returned.
+
+    my %h1 = 'apples' => 3, 'oranges' => 7;  # (no key type specified)
+    say %h1.keyof;                           # (Str(Any))
+
+    my %h2{Str} = 'oranges' => 7;            # (keys must be of type Str
+    say %h2.keyof;                           # (Str)
+    %h2{3} = 'apples';                       # throws exception of type X::TypeCheck::Binding
+
 =head1 Subscript Adverbs
 
 Some methods are implemented as adverbs on subscripts.