Add a warning to des.h.

DES being deprecated is hopefully well-established enough that no one is
going start complaining our implementation isn't constant-time. But I
think this is the only non-constant-time code left without a warning, so
add one for completeness.

(It is possible to implement DES with bitslicing, but 3DES TLS ciphers
are too slow as it is and hopefully not long for the world.)

Change-Id: I6b0de915e89ffe2d11372f7109642fcff44b11bf
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/43244
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

include/openssl/des.h

@@ -65,6 +65,12 @@ extern "C" {
 
 
 // DES.
+//
+// This module is deprecated and retained for legacy reasons only. It is slow
+// and may leak key material with timing or cache side channels. Moreover,
+// single-keyed DES is broken and can be brute-forced in under a day.
+//
+// Use a modern cipher, such as AES-GCM or ChaCha20-Poly1305, instead.
 
 
 typedef struct DES_cblock_st {