Document and implement the public API for when the backend doesn't support the requested algorithm
diff --git a/docs/hazmat/primitives/symmetric-encryption.rst b/docs/hazmat/primitives/symmetric-encryption.rst
index b8b1c83..48bad92 100644
--- a/docs/hazmat/primitives/symmetric-encryption.rst
+++ b/docs/hazmat/primitives/symmetric-encryption.rst
@@ -42,12 +42,21 @@
:class:`~cryptography.hazmat.primitives.interfaces.CipherContext`
provider.
+ If the backend doesn't support the requested combination of ``cipher``
+ and ``mode`` a :class:`cryptography.exceptions.NoSuchAlgorithm` will
+ be raised.
+
.. method:: decryptor()
:return: A decrypting
:class:`~cryptography.hazmat.primitives.interfaces.CipherContext`
provider.
+ If the backend doesn't support the requested combination of ``cipher``
+ and ``mode`` a :class:`cryptography.exceptions.NoSuchAlgorithm` will
+ be raised.
+
+
.. currentmodule:: cryptography.hazmat.primitives.interfaces
.. class:: CipherContext