Latex3: l3doc doesn't make "TF" grey when documenting internal conditionals

MWE

% \iffalse
%<*driver>
\documentclass{l3doc}
\begin{document}
\DocInput{\jobname.dtx}
\end{document}
%</driver>
% \fi
% \begin{implementation}
%    \begin{macrocode}
%<@@=testpkg>
%<*package>
%    \end{macrocode}
% \begin{macro}[TF]{\@@_true:}
% This function is interesting.
%    \begin{macrocode}
\prg_new_conditional:Nnn \@@_true: { T, F, TF }
  { \prg_return_true: }
%    \end{macrocode}
% \end{macro}
%    \begin{macrocode}
%</package>
%    \end{macrocode}
% \end{implementation}

"TF" should be gray, shouldn't it?