f clarify serialization change restriction comment

Author: TheBlueMatt

lightning/src/ln/msgs.rs

@@ -1231,13 +1231,15 @@ impl_writeable_msg!(UpdateFulfillHTLC, {
 }, {});
 
 // Note that this is written as a part of ChannelManager objects, and thus cannot change its
-// serialization format in a non-backwards-compatible way.
+// serialization format in a way which assumes we know the total serialized length/message end
+// position..
 impl_writeable!(OnionErrorPacket, {
 	data
 });
 
 // Note that this is written as a part of ChannelManager objects, and thus cannot change its
-// serialization format in a non-backwards-compatible way.
+// serialization format in a way which assumes we know the total serialized length/message end
+// position..
 impl Writeable for OnionPacket {
 	fn write<W: Writer>(&self, w: &mut W) -> Result<(), io::Error> {
 		self.version.write(w)?;