Knock the FFI documentation into some sort of shape for the release:

- The blurb about "the FFI is in two/three parts..." was repeated in
  three places.  We also at some point seem to have lost the property
  that the FFI spec is a self-contained document; I don't try to fix
  that here, since we're going to replace it with the new spec at some
  point.

- Replace references to Addr and ForeignObj with Ptr and ForeignPtr.

- Remove mentions of GHC's ByteArray and MutableByteArray types, as
  these are deprecated and will be removed in a couple of versions.
  (mostly subsumed by allocaBytes and friends).

- Catch up with GHC's removal of the library specification from foreign
  import.  Mention that libraries are specified in a compiler-dependent
  way now, and that GHC uses either packages or command-line opts for
  this.

- Fix up some markup.