update FFI documentation

[pnkfelix]

The Ffi documentation is out of date.

At the very least, it needs to incorporate documentation of the linux support that was added at some point.

(Lars said that Windows also worked at some point; but that's a seperate task.)

[pnkfelix]

Some rough starts in changeset:4545 and changeset:4546

[pnkfelix]

We should make sure we address David Rush's comments/complaints from:

https://lists.ccs.neu.edu/pipermail/larceny-users/2007-July/000093.html

https://lists.ccs.neu.edu/pipermail/larceny-users/2007-July/000097.html

[pnkfelix]

And https://lists.ccs.neu.edu/pipermail/larceny-users/2007-July/000091.html