message: Add comment to COIN constant.
  The COIN constant is critical in understanding Bitcoin's supply, but what it represents isn't clear from the name of the constant. Adding a comment clarifies the meaning of the constant for future readers.
diff: typedef int64_t CAmount;
  static constexpr CAmount COIN = 1000000;
  -static const CAmount CENT = 10000;
  +static constexpr CAmount CENT = 10000;
  static const CAmount MIN_TX_FEE_PREV7 = CENT;
  static const CAmount MIN_TX_FEE = CENT / 10;
  