docs: add a usage example for impl_writeable_tlv_based

lightning/src/util/ser_macros.rs

@@ -522,8 +522,8 @@ macro_rules! write_ver_prefix {
 	}
 }
 
-/// Writes out a suffix to an object which contains potentially backwards-compatible, optional
-/// fields which old nodes can happily ignore.
+/// Writes out a suffix to an object as a length-prefixed TLV stream which contains potentially
+/// backwards-compatible, optional fields which old nodes can happily ignore.
 ///
 /// It is written out in TLV format and, as with all TLV fields, unknown even fields cause a
 /// [`DecodeError::UnknownRequiredFeature`] error, with unknown odd fields ignored.
@@ -627,10 +627,28 @@ macro_rules! _init_and_read_tlv_fields {
 }
 
 /// Implements [`Readable`]/[`Writeable`] for a struct storing it as a set of TLVs
-/// If $fieldty is `required`, then $field is a required field that is not an Option nor a Vec.
-/// If $fieldty is `option`, then $field is optional field.
-/// if $fieldty is `vec_type`, then $field is a Vec, which needs to have its individual elements
-/// serialized.
+/// If `$fieldty` is `required`, then `$field` is a required field that is not an Option nor a Vec.
+/// If `$fieldty` is `(default_value, $default)`, then `$field` will be set to `$default` if not present.
+/// If `$fieldty` is `option`, then `$field` is optional field.
+/// If `$fieldty` is `vec_type`, then `$field` is a Vec, which needs to have its individual elements serialized.
+///
+/// For example,
+/// ```
+/// # use lightning::impl_writeable_tlv_based;
+/// struct LightningMessage {
+/// 	tlv_integer: u32,
+/// 	tlv_default_integer: u32,
+/// 	tlv_optional_integer: Option<u32>,
+/// 	tlv_vec_type_integer: Vec<u32>,
+/// }
+///
+/// impl_writeable_tlv_based!(LightningMessage, {
+/// 	(0, tlv_integer, required),
+/// 	(1, tlv_default_integer, (default_value, 7)),
+/// 	(2, tlv_optional_integer, option),
+/// 	(3, tlv_vec_type_integer, vec_type),
+/// });
+/// ```
 ///
 /// [`Readable`]: crate::util::ser::Readable
 /// [`Writeable`]: crate::util::ser::Writeable