add missing sign_x509_certificate X509Backend interface docs
diff --git a/docs/hazmat/backends/interfaces.rst b/docs/hazmat/backends/interfaces.rst
index fb3786c..b6b5976 100644
--- a/docs/hazmat/backends/interfaces.rst
+++ b/docs/hazmat/backends/interfaces.rst
@@ -550,6 +550,26 @@
:returns: A new object with the
:class:`~cryptography.x509.CertificateSigningRequest` interface.
+ .. method:: sign_x509_certificate(builder, private_key, algorithm)
+
+ .. versionadded:: 1.0
+
+ :param builder: An instance of
+ :class:`~cryptography.x509.CertificateBuilder`.
+
+ :param private_key: The
+ :class:`~cryptography.hazmat.primitives.asymmetric.rsa.RSAPrivateKey`,
+ :class:`~cryptography.hazmat.primitives.asymmetric.dsa.DSAPrivateKey` or
+ :class:`~cryptography.hazmat.primitives.asymmetric.ec.EllipticCurvePrivateKey`
+ that will be used to sign the certificate.
+
+ :param algorithm: The
+ :class:`~cryptography.hazmat.primitives.hashes.HashAlgorithm`
+ that will be used to generate the certificate signature.
+
+ :returns: A new object with the
+ :class:`~cryptography.x509.Certificate` interface.
+
.. class:: DHBackend