lavu/md5: Add doxy

Mostly a copy&paste from other hash functions, with changes
where required.
Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/md5.h

@@ -36,10 +36,42 @@ extern const int av_md5_size;
 
 struct AVMD5;
 
+/**
+ * Allocate an AVMD5 context.
+ */
 struct AVMD5 *av_md5_alloc(void);
+
+/**
+ * Initialize MD5 hashing.
+ *
+ * @param ctx pointer to the function context (of size av_md5_size)
+ */
 void av_md5_init(struct AVMD5 *ctx);
+
+/**
+ * Update hash value.
+ *
+ * @param ctx hash function context
+ * @param src input data to update hash with
+ * @param len input data length
+ */
 void av_md5_update(struct AVMD5 *ctx, const uint8_t *src, int len);
+
+/**
+ * Finish hashing and output digest value.
+ *
+ * @param ctx hash function context
+ * @param dst buffer where output digest value is stored
+ */
 void av_md5_final(struct AVMD5 *ctx, uint8_t *dst);
+
+/**
+ * Hash an array of data.
+ *
+ * @param dst The output buffer to write the digest into
+ * @param src The data to hash
+ * @param len The length of the data, in bytes
+ */
 void av_md5_sum(uint8_t *dst, const uint8_t *src, const int len);
 
 /**