Add documentation warnings for weak algorithms

MD2, MD4, MD5, DES and SHA-1 are considered weak and their use
constitutes a security risk. If possible, we recommend avoiding
dependencies on them, and considering stronger message digests and
ciphers instead.
diff --git a/include/polarssl/des.h b/include/polarssl/des.h
index 3f7236c..59538a3 100644
--- a/include/polarssl/des.h
+++ b/include/polarssl/des.h
@@ -20,6 +20,10 @@
  *  You should have received a copy of the GNU General Public License along
  *  with this program; if not, write to the Free Software Foundation, Inc.,
  *  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * \warning   DES is considered a weak cipher and its use constitutes a
+ *            security risk. We recommend considering stronger ciphers instead.
+ *
  */
 #ifndef POLARSSL_DES_H
 #define POLARSSL_DES_H
@@ -56,6 +60,10 @@
 
 /**
  * \brief          DES context structure
+ *
+ * \warning        DES is considered a weak cipher and its use constitutes a
+ *                 security risk. We recommend considering stronger ciphers
+ *                 instead.
  */
 typedef struct
 {
@@ -78,6 +86,10 @@
  * \brief          Initialize DES context
  *
  * \param ctx      DES context to be initialized
+ *
+ * \warning        DES is considered a weak cipher and its use constitutes a
+ *                 security risk. We recommend considering stronger ciphers
+ *                 instead.
  */
 void des_init( des_context *ctx );
 
@@ -85,6 +97,10 @@
  * \brief          Clear DES context
  *
  * \param ctx      DES context to be cleared
+ *
+ * \warning        DES is considered a weak cipher and its use constitutes a
+ *                 security risk. We recommend considering stronger ciphers
+ *                 instead.
  */
 void des_free( des_context *ctx );
 
@@ -109,6 +125,10 @@
  *                 a parity bit to allow verification.
  *
  * \param key      8-byte secret key
+ *
+ * \warning        DES is considered a weak cipher and its use constitutes a
+ *                 security risk. We recommend considering stronger ciphers
+ *                 instead.
  */
 void des_key_set_parity( unsigned char key[DES_KEY_SIZE] );
 
@@ -121,6 +141,10 @@
  * \param key      8-byte secret key
  *
  * \return         0 is parity was ok, 1 if parity was not correct.
+ *
+ * \warning        DES is considered a weak cipher and its use constitutes a
+ *                 security risk. We recommend considering stronger ciphers
+ *                 instead.
  */
 int des_key_check_key_parity( const unsigned char key[DES_KEY_SIZE] );
 
@@ -130,6 +154,10 @@
  * \param key      8-byte secret key
  *
  * \return         0 if no weak key was found, 1 if a weak key was identified.
+ *
+ * \warning        DES is considered a weak cipher and its use constitutes a
+ *                 security risk. We recommend considering stronger ciphers
+ *                 instead.
  */
 int des_key_check_weak( const unsigned char key[DES_KEY_SIZE] );
 
@@ -140,6 +168,10 @@
  * \param key      8-byte secret key
  *
  * \return         0
+ *
+ * \warning        DES is considered a weak cipher and its use constitutes a
+ *                 security risk. We recommend considering stronger ciphers
+ *                 instead.
  */
 int des_setkey_enc( des_context *ctx, const unsigned char key[DES_KEY_SIZE] );
 
@@ -150,6 +182,10 @@
  * \param key      8-byte secret key
  *
  * \return         0
+ *
+ * \warning        DES is considered a weak cipher and its use constitutes a
+ *                 security risk. We recommend considering stronger ciphers
+ *                 instead.
  */
 int des_setkey_dec( des_context *ctx, const unsigned char key[DES_KEY_SIZE] );
 
@@ -205,6 +241,10 @@
  * \param output   64-bit output block
  *
  * \return         0 if successful
+ *
+ * \warning        DES is considered a weak cipher and its use constitutes a
+ *                 security risk. We recommend considering stronger ciphers
+ *                 instead.
  */
 int des_crypt_ecb( des_context *ctx,
                     const unsigned char input[8],
@@ -228,6 +268,10 @@
  * \param iv       initialization vector (updated after use)
  * \param input    buffer holding the input data
  * \param output   buffer holding the output data
+ *
+ * \warning        DES is considered a weak cipher and its use constitutes a
+ *                 security risk. We recommend considering stronger ciphers
+ *                 instead.
  */
 int des_crypt_cbc( des_context *ctx,
                     int mode,