adjust language around cryptoperiod of keys

Co-authored-by: Orie Steele <[email protected]>

Author: mprorock

index.html

@@ -569,7 +569,9 @@ <h3>
         </p>
 
         <p>
-          Ephemeral keys MUST be used.
+          Consult <a href="https://csrc.nist.gov/publications/detail/sp/800-57-part-1/rev-5/final">
+            NIST SP 800-57
+          </a> for guidance on cryptoperiod, which is the time span during which a specific key is authorized for use or in which the keys for a given system or application may remain in effect.
         </p>
 
         <p>