Update QPY version 14 desc.

qiskit/qpy/__init__.py

@@ -375,7 +375,24 @@ def open(*args):
 Version 14
 ----------
 
-Version 14 added support for constant types to classical expressions.
+Version 14 adds support for additional :class:`~.types.Type` classes, and adds support for
+const-ness to existing types :class:`~.types.Bool` and :class:`~.types.Uint`.
+The ``EXPR_TYPE_BOOL`` and ``EXPR_TYPE_UNIT`` structs are now replaced by ``EXPR_TYPE_BOOL_V14``
+and ``EXPR_TYPE_UINT_V14``, respectively. See the updated expression type table below.
+
+EXPR_TYPE
+~~~~~~~~~
+
+A :class:`~.types.Type` is encoded by a single-byte ASCII ``char`` that encodes the kind of type,
+followed by a payload that varies depending on the type.  The defined codes are:
+
+======================  =========  =================================================================
+Qiskit class            Type code  Payload
+======================  =========  =================================================================
+:class:`~.types.Bool`   ``b``      One `_Bool const`.
+
+:class:`~.types.Uint`   ``u``      One ``uint32_t width``, followed by one ``_Bool const``.
+======================  =========  =================================================================
 
 .. _qpy_version_13: