add some missing docs for PKCS8SerializationBackend, update changelog
diff --git a/docs/hazmat/backends/openssl.rst b/docs/hazmat/backends/openssl.rst
index 6ad0d04..e829798 100644
--- a/docs/hazmat/backends/openssl.rst
+++ b/docs/hazmat/backends/openssl.rst
@@ -19,7 +19,9 @@
     * :class:`~cryptography.hazmat.backends.interfaces.HashBackend`
     * :class:`~cryptography.hazmat.backends.interfaces.HMACBackend`
     * :class:`~cryptography.hazmat.backends.interfaces.PBKDF2HMACBackend`
+    * :class:`~cryptography.hazmat.backends.interfaces.PKCS8SerializationBackend`
     * :class:`~cryptography.hazmat.backends.interfaces.RSABackend`
+    * :class:`~cryptography.hazmat.backends.interfaces.TraditionalOpenSSLSerializationBackend`
 
     It also exposes the following: