[Pair] small updates; mention Enum

lib/Pair.pod

@@ -2,6 +2,8 @@
 
 =head1 Pair
 
+    class Pair is Enum { ... }
+
 Consists of two parts, a I<key> and a I<value>. C<Pair>s can be seen as the
 atomic units in C<Hash>es, and they are also used in conjunction with named
 arguments and parameters.
@@ -12,6 +14,8 @@ There are three syntaxes for C<Pair>s:
     :key<value>         # ...means the same as this
     :$foo               # short for foo => $foo
 
+The immutable version of a C<Pair> is an C<Enum>.
+
 =head2 Methods
 
 =head3 key
@@ -22,7 +26,7 @@ Gives the I<key> part of the C<Pair>.
 
 =head3 value
 
-    multi method value(Pair:D:)
+    multi method value(Pair:D:) is rw
 
 Gives the I<value> part of the C<Pair>.