add comments

Author: smdn

src/MHZ19X_error_t.hpp

@@ -7,10 +7,14 @@
 enum class MHZ19X_error_t {
   // Represents a value that is neither success nor failure.
   none = -1,
+
+  // Represents that the operation was successfully completed.
   success = 0,
+
   uart_write_failure,
   uart_read_timeout,
   uart_read_failure,
+
   returnvalue_startbyte_mismatch,
   returnvalue_commandbyte_mismatch,
   returnvalue_checksum_mismatch,