Document all the parameters including cross references to specific providers where appropriate.
diff --git a/docs/hazmat/primitives/hmac.rst b/docs/hazmat/primitives/hmac.rst
index 10db40e..db5e98d 100644
--- a/docs/hazmat/primitives/hmac.rst
+++ b/docs/hazmat/primitives/hmac.rst
@@ -34,6 +34,16 @@
>>> 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.