Provide insight about the method Pair.freeze when dealing with Scalar.

Close #1728

Author: fluca1978

doc/Type/Pair.pod6

@@ -62,6 +62,18 @@ $v = 'value B';
 $pair.say; # OUTPUT: a => value B
 =end code
 
+It is possible to avoid such behaviour forcing the C<Pair> to remove the scalar container
+and to hold the effective value itself using the method L<freeze|/type/Pair#method_freeze>:
+
+=begin code
+my $v = 'value B';
+my $pair = a => $v;
+$pair.freeze;
+$v = 'value C';
+$pair.say; # OUTPUT: a => value B
+=end code
+
+
 =head1 Methods
 
 =head2 method new