docs: include distinct type declarations in codec module

Author: gertvdijk

docs/api/codec.md

@@ -20,6 +20,10 @@ SPDX-License-Identifier: CC0-1.0
 
 ## Data classes & types
 
+::: pykmp.codec.PhysicalBytes
+::: pykmp.codec.DataLinkBytes
+::: pykmp.codec.ApplicationBytes
+::: pykmp.codec.ApplicationDataBytes
 ::: pykmp.codec.ApplicationData
 ::: pykmp.codec.DataLinkData
 ::: pykmp.codec.PhysicalDirection

src/pykmp/codec.py

@@ -30,9 +30,16 @@
 
 
 PhysicalBytes = NewType("PhysicalBytes", bytes)
+"""Distinct type (bytes) representing serialized bytes on the physical layer."""
+
 DataLinkBytes = NewType("DataLinkBytes", bytes)
+"""Distinct type (bytes) representing serialized bytes on the data link layer."""
+
 ApplicationBytes = NewType("ApplicationBytes", bytes)
+"""Distinct type (bytes) representing serialized data on the application layer."""
+
 ApplicationDataBytes = NewType("ApplicationDataBytes", bytes)
+"""Distinct type (bytes) representing serialized application (message) data."""
 
 
 class AckReceivedException(Exception):  # noqa: N818