f: expand LengthReadable comment

Author: arik-so

lightning/src/util/ser.rs

@@ -361,8 +361,11 @@ where
 	fn read<R: LengthRead>(reader: &mut R, params: P) -> Result<Self, DecodeError>;
 }
 
-/// A trait that various higher-level LDK types implement allowing them to be read in
-/// from a [`Read`], requiring the implementer to provide the total length of the read.
+/// A trait that various LDK types implement allowing them to be read in from a [`Read`], requiring
+/// the implementer to provide the total length of the read.
+///
+/// Any type that implements [`Readable`] also automatically has a [`LengthReadable`]
+/// implementation, but some types, most notably onion packets, only implement [`LengthReadable`].
 pub trait LengthReadable
 where
 	Self: Sized,