man: fix description of splice offsets
The interface to splice(2) in io_uring differs in that it doesn't accept a
pointer for the offset values, but instead a literal loff_t. It then uses a
sentinel value of -1 to pass the equivalent of a NULL. This fixes
io_uring_enter.2 to clarify this.
@@ -549,11 +549,13 @@ system call.
.I splice_fd_in
is the file descriptor to read from,
.I splice_off_in
-is a pointer to an offset to read from,
+is an offset to read from,
.I fd
is the file descriptor to write to,
.I off
-is a pointer to an offset to from which to start writing to.
+is an offset from which to start writing to. A sentinel value of -1 is used
+to pass the equivalent of a NULL for the offsets to
+.BR splice(2).
.I len
contains the number of bytes to copy.
.I splice_flags
