Fix BZ # 2074

[Roland McGrath]

That does seem to make the documentation consistent with the code.  
The comment on __io_write_fn in libio/libio.h needs to be changed too.

Since that's the interface, it's unfortunate that we use ssize_t in the
type signatures.  They very much look like they are intended to be exactly
the interfaces of read and write (with the int fd replaced with the void
*cookie).  In fact, I'm quite sure that was my original intention in the
pre-libio stdio implementation.  If there's any way we can compatibly
recover the situation so the functions have the more-obvious interfaces,
that seems like the best thing to me.  But even in the interface described
by the __io_write_fn comment is not quite analogous to write's interface,
since write can return short counts.


Thanks,
Roland