propsys: mention that the functions are stubs

[Peter Davies]

> Wouldn't it be sufficient for our purposes to just count every first
> fixme in a WINAPI function as a stub? As it is now there's a large
> percentage of fixmes that don't include the 'stub' keyword.

A FIXME doesn't neccesarily indicate a stub. It sometimes is like
"Flag FOO unsupported", which is not the same as a stub. Maybe we
could have STUB macro to be called in stub functions.

Peter