Added backend check to cipher primitives
diff --git a/docs/hazmat/primitives/symmetric-encryption.rst b/docs/hazmat/primitives/symmetric-encryption.rst
index 741091b..71a1064 100644
--- a/docs/hazmat/primitives/symmetric-encryption.rst
+++ b/docs/hazmat/primitives/symmetric-encryption.rst
@@ -56,6 +56,10 @@
:class:`~cryptography.hazmat.backends.interfaces.CipherBackend`
provider.
+ :raises cryptography.exceptions.UnsupportedInterface: This is raised if the
+ provided ``backend`` does not implement
+ :class:`~cryptography.hazmat.backends.interfaces.CipherBackend`
+
.. method:: encryptor()
:return: An encrypting