-ffi poorly documented

[wightnoise]

The flag is not mentioned in the "using FFI in GHC"
section, and only once elsewhere, in the overall flags
section.  It was rather confusing
as to why ghc was choking on the import bit of "foreign
import ...".

Yes, it's only a one-time road-block for new users of
FFI, but it's a bit annoying that it wasn't obvious
that FFI features were only enabled when the -ffi flag
is passed.