Minor feature request: BOOST_PP_IGNORE or update docs for BOOST_PP_EMPTY

The current `BOOST_PP_EMPTY()` is not variadic, so its usefulness in `BOOST_PP_IF()` and the like is limited to when there are no arguments/params in the resultant call.

For example, this doesn't work:
```
#define X(a, b) result(a, b)
#define MACRO(a, b, c) BOOST_PP_IF(c, X, BOOST_PP_EMPTY)(a, b)
```

As explained in #34, changing the signature of `BOOST_PP_EMPTY()` is impractical, and another solution exists: `BOOST_VMD_EMPTY(...)` in `boost/vmd/empty.hpp`.

So this is a very minor issue, but it would be nice if either:
1. There were a `BOOST_PP_IGNORE(...)` to do this, or...
2. The reference docs for the `BOOST_PP_EMPTY` could also point to the VMD macro to use instead.