ref-manual/classes: document the cython class

Small class added in oecore by commit
64b6e8580842949be58f97f4dff91d1f097c4020 ("classes-recipe: add cython
class").

(From yocto-docs rev: 0979e02b71b6c91fa27cdeacd70036564fbe1a2d)

Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/classes.rst

@@ -614,6 +614,15 @@ You will find some more details in the
 ":ref:`dev-manual/vulnerabilities:checking for vulnerabilities`"
 section in the Development Tasks Manual.
 
+.. _ref-classes-cython:
+
+``cython``
+==========
+
+The :ref:`ref-classes-cython` class can be used by Python recipes that require
+`Cython <https://cython.org/>`__ as part of their build dependencies
+(:term:`DEPENDS`).
+
 .. _ref-classes-debian:
 
 ``debian``