add backoff_decorator

docs/order.md

@@ -130,7 +130,4 @@ Return profit in the account currency for a specified trading operation.
 | Type    | Description                       |
 |---------|-----------------------------------|
 | `float` | Returns float value if successful |
-#### Raises:
-| Exception    | Description       |
-|--------------|-------------------|
-| `OrderError` | If not successful |
+| `None`  | If not successful                 |

docs/symbol.md

@@ -165,7 +165,7 @@ This is a dummy method that returns the minimum volume of the symbol. It is mean
 async def currency_conversion(*, amount: float, base: str,
                               quote: str) -> float
 ```
-Convert from one currency to the other.
+Convert from one currency to the other. Returns the amount in terms of the base currency.
 #### Parameters
 | Name     | Type    | Description                                            | Default |
 |----------|---------|--------------------------------------------------------|---------|