Fix PR#5454: Update docs

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@11973 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02

Author: danmey

stdlib/digest.mli

@@ -18,7 +18,10 @@
    This module provides functions to compute 128-bit ``digests'' of
    arbitrary-length strings or files. The digests are of cryptographic
    quality: it is very hard, given a digest, to forge a string having
-   that digest. The algorithm used is MD5.
+   that digest. The algorithm used is MD5. This module should not be
+   used for secure and sensitive cryptographic applications. For these
+   kind of applications more recent and stronger cryptographic
+   primitives should be used instead.
 *)
 
 type t = string