Document Hash's :kv adverb

Also fix a small grammar error.

Author: Dan Zwell

doc/Type/Hash.pod6

@@ -674,12 +674,17 @@ The adverb C<:p> returns a C<Pair> or a List of C<Pair> instead of just the valu
 
 =head2 C<:v> and C<:k>
 
-The adverbs C<:v> and C<:k> return the key or value or a list their of.
+The adverbs C<:v> and C<:k> return the key or value or a list thereof.
 
     my %h = a => 1, b => 2;
     say %h<a>:k;    # OUTPUT: «a␤»
     say %h<a b>:k;  # OUTPUT: «(a b)␤»
 
+The adverb C<:kv> returns a list of keys and values.
+
+    my %h = a => 1, b => 2, c => 3;
+    say %h<a c>:kv;  # OUTPUT: «(a 1 c 3)␤»
+
 You can also use the adverbs without knowing anything about the hash by using
 empty angle brackets in which case all the keys and values will be listed: