uucore::pipes::splice_exact: improve document

[oech3]

I previously said this function is dangerous since it does not return bytes sent. But actually, we can make pipe empty when splice_exact() failed by reading the pipe until EOF.

But add document to avoid misreading.

Also this function does not panic.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/resolution (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/cut/cut-huge-range is now passing!