Clarify that the SipHash implementation has not be validated for cryptographic purposes.

Author: Palmer Cox

src/libstd/hash.rs

@@ -15,8 +15,13 @@
  *
  * Consider this as a main "general-purpose" hash for all hashtables: it
  * runs at good speed (competitive with spooky and city) and permits
- * cryptographically strong _keyed_ hashing. Key your hashtables from a
- * CPRNG like rand::rng.
+ * strong _keyed_ hashing. Key your hashtables from a strong RNG,
+ * such as rand::rng.
+ *
+ * Although the SipHash algorithm is considered to be cryptographically
+ * strong, this implementation has not been reviewed for such purposes.
+ * As such, all cryptographic uses of this implementation are strongly
+ * discouraged.
  */
 
 #[allow(missing_doc)];