1 files changed, 25 insertions, 4 deletions
diff --git a/docs/hazmat/primitives/hmac.rst b/docs/hazmat/primitives/hmac.rst
index bd1a4934..db5e98d3 100644
--- a/docs/hazmat/primitives/hmac.rst
+++ b/docs/hazmat/primitives/hmac.rst
@@ -15,7 +15,7 @@ message authentication codes using a cryptographic hash function coupled with a
 secret key. You can use an HMAC to verify integrity as well as authenticate a
 message.
 
-.. class:: HMAC(key, algorithm)
+.. class:: HMAC(key, algorithm, backend)
 
     HMAC objects take a ``key`` and a provider of
     :class:`~cryptography.hazmat.primitives.interfaces.HashAlgorithm`.
@@ -27,24 +27,45 @@ message.
 
     .. doctest::
 
+        >>> from cryptography.hazmat.bindings import default_backend
         >>> from cryptography.hazmat.primitives import hashes, hmac
-        >>> h = hmac.HMAC(key, hashes.SHA256())
+        >>> h = hmac.HMAC(key, hashes.SHA256(), backend=default_backend())
         >>> h.update(b"message to hash")
         >>> h.finalize()
         '#F\xdaI\x8b"e\xc4\xf1\xbb\x9a\x8fc\xff\xf5\xdex.\xbc\xcd/+\x8a\x86\x1d\x84\'\xc3\xa6\x1d\xd8J'
 
+
+    :param key: Secret key as ``bytes``.
+    :param algorithm: A
+        :class:`~cryptography.hazmat.primitives.interfaces.HashAlgorithm`
+        provider such as those described in
+        :ref:`Cryptographic Hashes <cryptographic-hash-algorithms>`.
+    :param backend: A
+        :class:`~cryptography.hazmat.bindings.interfaces.HMACBackend`
+        provider.
+
     .. method:: update(msg)
 
         :param bytes msg: The bytes to hash and authenticate.
+        :raises cryptography.exceptions.AlreadyFinalized: See :meth:`finalize`
 
     .. method:: copy()
 
-        :return: a new instance of this object with a copied internal state.
+        Copy this :class:`HMAC` instance, usually so that we may call
+        :meth:`finalize` and get an intermediate digest value while we continue
+        to call :meth:`update` on the original.
+
+        :return: A new instance of :class:`HMAC` which can be updated
+            and finalized independently of the original instance.
+        :raises cryptography.exceptions.AlreadyFinalized: See :meth:`finalize`
 
     .. method:: finalize()
 
         Finalize the current context and return the message digest as bytes.
 
-        Once ``finalize`` is called this object can no longer be used.
+        Once ``finalize`` is called this object can no longer be used and
+        :meth:`update`, :meth:`copy`, and :meth:`finalize` will raise
+        :class:`~cryptography.exceptions.AlreadyFinalized`.
 
         :return bytes: The message digest as bytes.
+        :raises cryptography.exceptions.AlreadyFinalized: