Be explicit with space assignment in documentation

docs/hyperopt.md

@@ -249,11 +249,11 @@ We continue to define hyperoptable parameters:
 
 ```python
 class MyAwesomeStrategy(IStrategy):
-    buy_adx = IntParameter(20, 40, default=30)
-    buy_rsi = IntParameter(20, 40, default=30)
-    buy_adx_enabled = CategoricalParameter([True, False])
-    buy_rsi_enabled = CategoricalParameter([True, False])
-    buy_trigger = CategoricalParameter(['bb_lower', 'macd_cross_signal'])
+    buy_adx = IntParameter(20, 40, default=30, space="buy")
+    buy_rsi = IntParameter(20, 40, default=30, space="buy")
+    buy_adx_enabled = CategoricalParameter([True, False], space="buy")
+    buy_rsi_enabled = CategoricalParameter([True, False], space="buy")
+    buy_trigger = CategoricalParameter(['bb_lower', 'macd_cross_signal'], space="buy")
 ```
 
 Above definition says: I have five parameters I want to randomly combine to find the best combination.
@@ -262,6 +262,10 @@ Then we have three category variables. First two are either `True` or `False`.
 We use these to either enable or disable the ADX and RSI guards.
 The last one we call `trigger` and use it to decide which buy trigger we want to use.
 
+!!! Note "Parameter space assignment"
+    Parameters must either be assigned to a variable named `buy_*` or `sell_*` - or contain `space='buy'` | `space='sell'` to be assigned to a space correctly.
+    If no parameter is available for a space, you'll receive the error that no space was found when running hyperopt.
+
 So let's write the buy strategy using these values:
 
 ```python