codec: document UdpFramed decoder errors (#8248)

tokio-util/src/udp/frame.rs

@@ -22,6 +22,12 @@ use std::{
 /// handle encoding and decoding of messages frames. Note that the incoming and
 /// outgoing frame types may be distinct.
 ///
+/// A single datagram may decode into multiple frames. `UdpFramed` will keep
+/// calling [`Decoder::decode_eof`] with the current datagram until it returns
+/// `Ok(None)`. If a decoder wants to discard a malformed datagram and continue
+/// receiving later datagrams, it should consume or clear the remaining bytes
+/// from the buffer before returning `Err`.
+///
 /// This function returns a *single* object that is both [`Stream`] and [`Sink`];
 /// grouping this into a single object is often useful for layering things which
 /// require both read and write access to the underlying object.