Add comments to CustomUintFormatter

Author: sipa

src/serialize.h

@@ -520,6 +520,15 @@ struct VarIntFormatter
     }
 };
 
+/** Serialization wrapper class for custom integers and enums.
+ *
+ * It permits specifying the serialized size (1 to 8 bytes) and endianness.
+ *
+ * Use the big endian mode for values that are stored in memory in native
+ * byte order, but serialized in big endian notation. This is only intended
+ * to implement serializers that are compatible with existing formats, and
+ * its use is not recommended for new data structures.
+ */
 template<int Bytes, bool BigEndian = false>
 struct CustomUintFormatter
 {