Extend Pair description

Author: Altai-man

doc/Type/Pair.pod6

@@ -97,12 +97,16 @@ $pair.say; # OUTPUT: «a => value B␤»
 =end code
 
 As Pair implements L<Associative|/type/Associative> role, its value can be
-accessed using Associative subscription operation. Subscrip adverbs work as well.
+accessed using Associative subscription operator, however, due to X<Pair>'s
+singular nature, the pair's value will be only returned for the pair's key.
+L<Nil|/type/Nil> object will be returned for any other key. Subscript
+adverbs such as X<:exists> can be used on Pair.
 
 =for code
 my $pair = a => 5;
-say $pair<a>;        # OUTPUT: «5␤»
-say $pair<a>:exists; # OUTPUT: «True␤»
+say $pair<a>;           # OUTPUT: «5␤»
+say $pair<a>:exists;    # OUTPUT: «True␤»
+say $pair<no-such-key>; # OUTPUT: «Nil␤»
 
 =head1 Methods