[Pair] elaborate on literals; add X<> anchors

Author: moritz

lib/Pair.pod

@@ -8,12 +8,18 @@ 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.
 
+X<:> X<< => >> X<:!>
 There are three syntaxes for C<Pair>s:
 
     'key' => 'value'    # this...
     :key<value>         # ...means the same as this
     :$foo               # short for foo => $foo
 
+Variants of this are
+
+    :key                # same as   key => True
+    :!key               # same as   key => False
+
 The immutable version of a C<Pair> is an C<Enum>.
 
 =head1 Methods