Missing crucial information in splice(2) manpage

[Absee Seeab <doesnt.look.like.temp.mail@xxxxxxxxx>]

Hello,

The splice(2) manpage (at least the version at man7.org) does not
specify that off_in/off_out are written to after the operation.

This, however, is done by the kernel (observed both by behaviour and
source code).

Plus I see similar functions (like copy_file_range and send_file) do
document this fact, but splice does not, so it seems like a
documentation omission.

(This caused me a bit of a debugging headache today)

Thanks