So, here is a silly question, should -$ be undocumented now:
2002-08-09 Neil Booth <neil@daikokuya.co.uk>
* gcc.c (cpp_unique_options): Remove -$.
invoke.texi:
@item Preprocessor Options
@xref{Preprocessor Options,,Options Controlling the Preprocessor}.
@gccoptlist{
-$
?