DOC: Document when ts_ref is set

Author: threecgreen

include/databento/enums.hpp

@@ -205,7 +205,8 @@ enum StatType : std::uint16_t {
   IndicativeOpeningPrice = 2,
   // The settlement price of an instrument. `price` will be set and `flags`
   // indicate whether the price is final or preliminary and actual or
-  // theoretical.
+  // theoretical. `ts_ref` will indicate the trading date of the settlement
+  // price.
   SettlementPrice = 3,
   // The lowest trade price of an instrument during the trading session.
   // `price` will be set.
@@ -214,7 +215,8 @@ enum StatType : std::uint16_t {
   // `price` will be set.
   TradingSessionHighPrice = 5,
   // The number of contracts cleared for an instrument on the previous trading
-  // date. `quantity` will be set.
+  // date. `quantity` will be set. `ts_ref` will indicate the trading date of
+  // the volume.
   ClearedVolume = 6,
   // The lowest offer price for an instrument during the trading session.
   // `price` will be set.
@@ -223,7 +225,8 @@ enum StatType : std::uint16_t {
   // `price` will be set.
   HighestBid = 8,
   // The current number of outstanding contracts of an instrument. `quantity`
-  // will be set.
+  // will be set. `ts_ref` will indicate the trading date for which the open
+  // interest was calculated.
   OpenInterest = 9,
   // The volume-weighted average price (VWAP) for a fixing period. `price` will
   // be set.