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.

include/mbedtls/md.h

@@ -44,6 +44,14 @@
 extern "C" {
 #endif
 
+/**
+ * \brief     Enumeration of supported message digests
+ *
+ * \warning   MD2, MD4, MD5 and SHA-1 are considered weak message digests and
+ *            their use constitutes a security risk. We recommend considering
+ *            stronger message digests instead.
+ *
+ */
 typedef enum {
     MBEDTLS_MD_NONE=0,
     MBEDTLS_MD_MD2,