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/mbedtls/md4.h b/include/mbedtls/md4.h
index 45214d4..b02d4b6 100644
--- a/include/mbedtls/md4.h
+++ b/include/mbedtls/md4.h
@@ -19,6 +19,11 @@
  *  limitations under the License.
  *
  *  This file is part of mbed TLS (https://tls.mbed.org)
+ *
+ * \warning   MD4 is considered a weak message digest and its use constitutes a
+ *            security risk. We recommend considering stronger message
+ *            digests instead.
+ *
  */
 #ifndef MBEDTLS_MD4_H
 #define MBEDTLS_MD4_H
@@ -42,6 +47,11 @@
 
 /**
  * \brief          MD4 context structure
+ *
+ * \warning        MD4 is considered a weak message digest and its use
+ *                 constitutes a security risk. We recommend considering
+ *                 stronger message digests instead.
+ *
  */
 typedef struct
 {
@@ -55,6 +65,11 @@
  * \brief          Initialize MD4 context
  *
  * \param ctx      MD4 context to be initialized
+ *
+ * \warning        MD4 is considered a weak message digest and its use
+ *                 constitutes a security risk. We recommend considering
+ *                 stronger message digests instead.
+ *
  */
 void mbedtls_md4_init( mbedtls_md4_context *ctx );
 
@@ -62,6 +77,11 @@
  * \brief          Clear MD4 context
  *
  * \param ctx      MD4 context to be cleared
+ *
+ * \warning        MD4 is considered a weak message digest and its use
+ *                 constitutes a security risk. We recommend considering
+ *                 stronger message digests instead.
+ *
  */
 void mbedtls_md4_free( mbedtls_md4_context *ctx );
 
@@ -70,6 +90,11 @@
  *
  * \param dst      The destination context
  * \param src      The context to be cloned
+ *
+ * \warning        MD4 is considered a weak message digest and its use
+ *                 constitutes a security risk. We recommend considering
+ *                 stronger message digests instead.
+ *
  */
 void mbedtls_md4_clone( mbedtls_md4_context *dst,
                         const mbedtls_md4_context *src );
@@ -78,6 +103,10 @@
  * \brief          MD4 context setup
  *
  * \param ctx      context to be initialized
+ *
+ * \warning        MD4 is considered a weak message digest and its use
+ *                 constitutes a security risk. We recommend considering
+ *                 stronger message digests instead.
  */
 void mbedtls_md4_starts( mbedtls_md4_context *ctx );
 
@@ -87,6 +116,11 @@
  * \param ctx      MD4 context
  * \param input    buffer holding the  data
  * \param ilen     length of the input data
+ *
+ * \warning        MD4 is considered a weak message digest and its use
+ *                 constitutes a security risk. We recommend considering
+ *                 stronger message digests instead.
+ *
  */
 void mbedtls_md4_update( mbedtls_md4_context *ctx, const unsigned char *input, size_t ilen );
 
@@ -95,6 +129,11 @@
  *
  * \param ctx      MD4 context
  * \param output   MD4 checksum result
+ *
+ * \warning        MD4 is considered a weak message digest and its use
+ *                 constitutes a security risk. We recommend considering
+ *                 stronger message digests instead.
+ *
  */
 void mbedtls_md4_finish( mbedtls_md4_context *ctx, unsigned char output[16] );
 
@@ -116,6 +155,11 @@
  * \param input    buffer holding the  data
  * \param ilen     length of the input data
  * \param output   MD4 checksum result
+ *
+ * \warning        MD4 is considered a weak message digest and its use
+ *                 constitutes a security risk. We recommend considering
+ *                 stronger message digests instead.
+ *
  */
 void mbedtls_md4( const unsigned char *input, size_t ilen, unsigned char output[16] );
 
@@ -123,6 +167,11 @@
  * \brief          Checkup routine
  *
  * \return         0 if successful, or 1 if the test failed
+ *
+ * \warning        MD4 is considered a weak message digest and its use
+ *                 constitutes a security risk. We recommend considering
+ *                 stronger message digests instead.
+ *
  */
 int mbedtls_md4_self_test( int verbose );