update docs re: PBKDF2HMAC iterations
diff --git a/docs/hazmat/primitives/key-derivation-functions.rst b/docs/hazmat/primitives/key-derivation-functions.rst
index e652ecb..bf069fa 100644
--- a/docs/hazmat/primitives/key-derivation-functions.rst
+++ b/docs/hazmat/primitives/key-derivation-functions.rst
@@ -58,7 +58,9 @@
:param bytes salt: A salt. `NIST SP 800-132`_ recommends 128-bits or
longer.
:param int iterations: The number of iterations to perform of the hash
- function. See OWASP's `Password Storage Cheat Sheet`_ for more
+ function. This can be used to control the length of time the operation
+ takes. Higher numbers help mitigate brute force attacks against derived
+ keys. See OWASP's `Password Storage Cheat Sheet`_ for more
detailed recommendations if you intend to use this for password storage.
:param backend: A
:class:`~cryptography.hazmat.backends.interfaces.CipherBackend`