Switch this to a warning block
diff --git a/docs/hazmat/primitives/key-derivation-functions.rst b/docs/hazmat/primitives/key-derivation-functions.rst
index da47ccd..d8a0e24 100644
--- a/docs/hazmat/primitives/key-derivation-functions.rst
+++ b/docs/hazmat/primitives/key-derivation-functions.rst
@@ -128,7 +128,10 @@
`HKDF`_ (HMAC-based Extract-and-Expand Key Derivation Function) is suitable
for deriving keys of a fixed size used for other cryptographic operations.
- **HKDF should not be used for password storage.**
+
+ .. warning::
+
+ HKDF should not be used for password storage.
.. doctest::