balanceHistory: update docs

docs/api.md

@@ -656,9 +656,37 @@ Query parameters:
 - *to*: specifies an end date as a Unix timestamp
 
 The optional query parameters:
-- *fiatcurrency*: if specified, the response will contain fiat rate at the time of transaction.
+- *fiatcurrency*: if specified, the response will contain fiat rate at the time of transaction. If not, all available currencies will be returned.
 - *groupBy*: an interval in seconds, to group results by. Default is 3600 seconds.
 
+Example response (fiatcurrency not specified):
+```javascript
+[
+  {
+    "time": 1578391200,
+    "txs": 5,
+    "received": "5000000",
+    "sent": "0",
+    "rates": {
+      "usd": 7855.9,
+      "eur": 6838.13,
+      ...
+    }
+  },
+  {
+    "time": 1578488400,
+    "txs": 1,
+    "received": "0",
+    "sent": "5000000",
+    "rates": {
+      "usd": 8283.11,
+      "eur": 7464.45,
+      ...
+    }
+  }
+]
+```
+
 Example response (fiatcurrency=usd):
 
 ```javascript