f clarify wording

Author: TheBlueMatt

lightning/src/util/ser.rs

@@ -389,7 +389,8 @@ impl Readable for BigSize {
 /// length integer here which is backwards-compatible but treats 0xffff as "read eight more bytes".
 ///
 /// To ensure we only have one valid encoding per value, we add 0xffff to values written as eight
-/// bytes, representing 0xffff as zero-eight-bytes, which 0xfffe is 0xfffe-two-bytes.
+/// bytes. Thus, 0xfffe is serialized as 0xfffe, whereas 0xffff is serialized as
+/// 0xffff0000000000000000 (i.e. read-eight-bytes then zero).
 struct U16OrU64(pub u64);
 impl Writeable for U16OrU64 {
 	#[inline]