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/md2.h b/include/mbedtls/md2.h
index 0f93fbf..d516042 100644
--- a/include/mbedtls/md2.h
+++ b/include/mbedtls/md2.h
@@ -19,6 +19,11 @@
  *  limitations under the License.
  *
  *  This file is part of mbed TLS (https://tls.mbed.org)
+ *
+ * \warning   MD2 is considered a weak message digest and its use constitutes a
+ *            security risk. We recommend considering stronger message
+ *            digests instead.
+ *
  */
 #ifndef MBEDTLS_MD2_H
 #define MBEDTLS_MD2_H
@@ -41,6 +46,11 @@
 
 /**
  * \brief          MD2 context structure
+ *
+ * \warning        MD2 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 MD2 context
  *
  * \param ctx      MD2 context to be initialized
+ *
+ * \warning        MD2 is considered a weak message digest and its use
+ *                 constitutes a security risk. We recommend considering
+ *                 stronger message digests instead.
+ *
  */
 void mbedtls_md2_init( mbedtls_md2_context *ctx );
 
@@ -62,6 +77,11 @@
  * \brief          Clear MD2 context
  *
  * \param ctx      MD2 context to be cleared
+ *
+ * \warning        MD2 is considered a weak message digest and its use
+ *                 constitutes a security risk. We recommend considering
+ *                 stronger message digests instead.
+ *
  */
 void mbedtls_md2_free( mbedtls_md2_context *ctx );
 
@@ -70,6 +90,11 @@
  *
  * \param dst      The destination context
  * \param src      The context to be cloned
+ *
+ * \warning        MD2 is considered a weak message digest and its use
+ *                 constitutes a security risk. We recommend considering
+ *                 stronger message digests instead.
+ *
  */
 void mbedtls_md2_clone( mbedtls_md2_context *dst,
                         const mbedtls_md2_context *src );
@@ -78,6 +103,11 @@
  * \brief          MD2 context setup
  *
  * \param ctx      context to be initialized
+ *
+ * \warning        MD2 is considered a weak message digest and its use
+ *                 constitutes a security risk. We recommend considering
+ *                 stronger message digests instead.
+ *
  */
 void mbedtls_md2_starts( mbedtls_md2_context *ctx );
 
@@ -87,6 +117,11 @@
  * \param ctx      MD2 context
  * \param input    buffer holding the  data
  * \param ilen     length of the input data
+ *
+ * \warning        MD2 is considered a weak message digest and its use
+ *                 constitutes a security risk. We recommend considering
+ *                 stronger message digests instead.
+ *
  */
 void mbedtls_md2_update( mbedtls_md2_context *ctx, const unsigned char *input, size_t ilen );
 
@@ -95,6 +130,11 @@
  *
  * \param ctx      MD2 context
  * \param output   MD2 checksum result
+ *
+ * \warning        MD2 is considered a weak message digest and its use
+ *                 constitutes a security risk. We recommend considering
+ *                 stronger message digests instead.
+ *
  */
 void mbedtls_md2_finish( mbedtls_md2_context *ctx, unsigned char output[16] );
 
@@ -116,6 +156,11 @@
  * \param input    buffer holding the  data
  * \param ilen     length of the input data
  * \param output   MD2 checksum result
+ *
+ * \warning        MD2 is considered a weak message digest and its use
+ *                 constitutes a security risk. We recommend considering
+ *                 stronger message digests instead.
+ *
  */
 void mbedtls_md2( const unsigned char *input, size_t ilen, unsigned char output[16] );
 
@@ -123,6 +168,11 @@
  * \brief          Checkup routine
  *
  * \return         0 if successful, or 1 if the test failed
+ *
+ * \warning        MD2 is considered a weak message digest and its use
+ *                 constitutes a security risk. We recommend considering
+ *                 stronger message digests instead.
+ *
  */
 int mbedtls_md2_self_test( int verbose );