Document other two parameters from sign method

- Remove incorrect CertificateBuilder doc-string
- Check that serial numbers are non-negative and < 160 bits
- Check that dates passed aren't earlier than the unix epoch
- Remove version from CertificateBuilder.__init__ and version method
diff --git a/docs/x509/reference.rst b/docs/x509/reference.rst
index 5a80984..b7fef94 100644
--- a/docs/x509/reference.rst
+++ b/docs/x509/reference.rst
@@ -455,6 +455,17 @@
 
         Sign the certificate using the CA's private key.
 
+        :param backend: Backend that will be used to build the certificate.
+            Must support the
+            :class:`~cryptography.hazmat.backends.interfaces.X509Backend`
+            interface.
+
+        :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 signature.