doc: Document API changes for ApplyEffect.

Author: heinezen

doc/nyan/api_reference/reference_util.md

@@ -54,6 +54,18 @@ Generalization object for conditions that can be used in `XORGate` nodes.
 **node**
 Node that is visited when the condition is true.
 
+## util.activity.condition.type.AbilityUsable
+
+```python
+AbilityUsable(Condition):
+    ability : abstract(Ability)
+```
+
+Is true when an ability can be used by the game entity when the node is visited.
+
+**ability**
+Ability definition used for the usability check. This can reference a specific ability of the game entity or an abstract API object from the `engine.ability.type` namespace. If a specific ability is referenced, the ability must be assigned to the game entity **and** be enabled for the check to pass. If an API object is referenced, at least one ability of the same type must be enabled for the check to pass.
+
 ## util.activity.condition.type.CommandInQueue
 
 ```python
@@ -87,7 +99,7 @@ Is true when the target of the next command in the game entity's command queue i
 **ability**
 Ability definition used for the range check.
 
-This can reference a specific ability of the game entity or an abstract API object from the `engine.ability.type` namespace. If a specific ability is referenced, the ability must be assigned to the game entity and must not be disabled. Otherwise, the range check fails. If an API object is referenced, the first active ability with the same type as the API object is executed.
+This can reference a specific ability of the game entity or an abstract API object from the `engine.ability.type` namespace. If a specific ability is referenced, the ability must be assigned to the game entity and must be enabled. Otherwise, the range check fails. If an API object is referenced, the first active ability with the same type as the API object is executed.
 
 If the ability has the property `Ranged`, the attributes of this property are utilized for the range check calculations. If the ability does not have a `Ranged` property, the condition is only true when the game entity is at the same position as the target.
 
@@ -159,7 +171,7 @@ Next node in the activity graph.
 **ability**
 Ability that is executed.
 
-This can reference a specific ability of the game entity or an abstract API object from the `engine.ability.type` namespace. If a specific ability is referenced, the ability must be assigned to the game entity and must not be disabled. Otherwise, the ability is not executed. If an API object is referenced, the first active ability with the same type as the API object is executed.
+This can reference a specific ability of the game entity or an abstract API object from the `engine.ability.type` namespace. If a specific ability is referenced, the ability must be assigned to the game entity and must be enabled. Otherwise, the ability is not executed. If an API object is referenced, the first active ability with the same type as the API object is executed.
 
 ## util.activity.node.type.End