Add documentation for the new IO.mode method

This requires Rakudo PR #773, which implements RT #125489

doc/Type/IO/Path.pod

@@ -510,6 +510,13 @@ or more readably:
     my $change_time = DateTime.new($chnge_instant);
     say $change_time;         # e.g. 2015-02-16T12:18:50Z
 
+=head2 method mode
+
+Return an L<IntStr> object representing the POSIX permissions of a file.
+
+    say ~"path/to/file".IO.mode;        #  e.g. '0644'
+    say +"path/to/file".IO.mode;        #  e.g. 420, where sprintf('%04o', 420) eq '0644'
+
 =head1 Related roles and classes
 
 See also the related role L<IO> and the related class L<IO::Handle>.

doc/Type/IO/Special.pod

@@ -111,4 +111,10 @@ The last accessed time always returns an undefined L<Instant>
 
 The last changed time always returns an undefined L<Instant>
 
+=head2 method mode
+
+    method mode(IO::Special:D:) returns Nil
+
+The mode always returns L<Nil>
+
 =end pod