Add an extra note about "use feature ':all'"

Author: leonerd

lib/feature.pm

@@ -1008,6 +1008,10 @@ =head2 The 'all' feature
 condition, with short-circuiting behaviour as soon as it finds one that does
 not.
 
+B<Note:> remember that this enables one specific feature whose name is C<all>;
+it does not enable all of the features.  This is not C<use feature ':all'>.
+For that, see the section below.
+
 =head1 FEATURE BUNDLES
 
 It's possible to load multiple features together, using