This patch minor cleans up ffi.h.in comments in a minor way. It fixes
some typos and capitalizations, adds some periods, and reformats some
comments to a more GNU-ish style. It also fixes up some stale
documentation.
An earlier patch added --disable-docs, but went too far, making it
impossible to build the docs.
It turns out that Automake seemingly has a bug preventing the
conditional build of an info file. So, this patch works around the
bug by putting the info_TEXINFOS rule into a new doc/Makefile.am.
Tested by building with and without --disable-docs and looking for the
existence of doc/libffi.info.