man/io_uring_prep_read.3: Explain unsigned input vs. __s32 output

io_uring_prep_read() reads an unsigned number of bytes,
and for short reads it returns the actual number of bytes read.
But the return value is __s32, so not all possible values
can be represented.

Explain this doesn't matter because of limits in the underlying syscall.

Signed-off-by: Andrew Sayers <andrew-github.com@pileofstuff.org>