added minor clarifications to the Manual

wiki/Manual.md

@@ -1802,6 +1802,10 @@ Most of methods returning orders within ccxt unified API will usually yield an o
 - The `lastTradeTimestamp` timestamp may have no value and may be `undefined/None/null` where not supported by the exchange or in case of an open order (an order that has not been filled nor partially filled yet).
 - The `lastTradeTimestamp`, if any, designates the timestamp of the last trade, in case the order is filled fully or partially, otherwise `lastTradeTimestamp` is `undefined/None/null`.
 - Order `status` prevails or has precedence over the `lastTradeTimestamp`.
+- The `cost` of an order is:
+  - `if (status === 'open' and filled === 0) { amount * price }`
+  - `if (status === 'closed' || status === 'canceled') { filled * price }`
+- The `cost` of an order means the total *quote* volume of the order (whereas the `amount` is the *base* volume). The value of `cost` should be as close to the actual most recent known order cost as possible. The `cost` field itself is there mostly for convenience and can be deduced from other fields.
 
 ### Placing Orders
 
@@ -2040,9 +2044,9 @@ Returns ordered array `[]` of trades (most recent trade last).
 }
 ```
 
-**The work on `'fee'` info is still in progress, fee info may be missing partially or entirely, depending on the exchange capabilities.**
-
-**The `fee` currency may be different from both traded currencies (for example, an ETH/BTC order with fees in USD).**
+- The work on `'fee'` info is still in progress, fee info may be missing partially or entirely, depending on the exchange capabilities.
+- The `fee` currency may be different from both traded currencies (for example, an ETH/BTC order with fees in USD).
+- The `cost` of the trade means `amount * price`. It is the total *quote* volume of the trade (whereas `amount` is the *base* volume). The cost field itself is there mostly for convenience and can be deduced from other fields.
 
 ### Trades By Order Id