update the doc of shrink

- remove what JSON::XS actually shrink
- mention what it does now might change in the future

lib/JSON/PP.pm

@@ -2154,15 +2154,13 @@ into the corresponding C<< $WIDGET{<id>} >> object:
     
     $enabled = $json->get_shrink
 
-In JSON::XS, this flag resizes strings generated by either
-C<encode> or C<decode> to their minimum size possible.
-It will also try to downgrade any strings to octet-form if possible.
+If C<$enable> is true (or missing), the string returned by C<encode> will
+be shrunk (i.e. downgraded if possible).
 
-In JSON::PP, it is noop about resizing strings but tries
-C<utf8::downgrade> to the returned string by C<encode>.
-See L<utf8>.
+The actual definition of what shrink does might change in future versions,
+but it will always try to save space at the expense of time.
 
-See L<JSON::XS/OBJECT-ORIENTED INTERFACE>.
+If C<$enable> is false, then JSON::PP does nothing.
 
 =head2 max_depth