[plt-scheme] Addition to ffi documentation

The explanation for ffi-callback does not clearly explain how to use the 
result of this procedure. It does return a _pointer but more accurately 
it returns an _fpointer. Could the docs be changed to something like:

This operation is the symmetric counterpart of |ffi-call|. It receives a 
Scheme procedure and creates a callback object, which can also be used 
as a pointer. This object can be used as a C-callable function, of type 
_fpointer, which will invoke |proc| using the types to specify how 
values are marshaled.