hashes/sha3: add missing macro documentation

sys/include/hashes/sha3.h

@@ -33,8 +33,19 @@
 extern "C" {
 #endif
 
+/**
+ * @brief   Length of SHA256 digests in bytes
+ */
 #define SHA3_256_DIGEST_LENGTH 32
+
+/**
+ * @brief   Length of SHA384 digests in bytes
+ */
 #define SHA3_384_DIGEST_LENGTH 48
+
+/**
+ * @brief   Length of SHA512 digests in bytes
+ */
 #define SHA3_512_DIGEST_LENGTH 64
 
 /**