Update docs

docs/api/creation.md

@@ -6,7 +6,7 @@
 
 Creates a new UI element, applying any given properties.
 
-- ### Type
+- **Type**
 
     ```lua
     function create(class: string): (Properties) -> Instance
@@ -15,7 +15,7 @@ Creates a new UI element, applying any given properties.
     type Properties = Map<string|number, any>
     ```
 
-- ### Details
+- **Details**
 
     The function can take either a `string` or an `Instance` as its first argument.
 
@@ -26,22 +26,22 @@ Creates a new UI element, applying any given properties.
     This returns another function that is used to apply any properties to the new
     instance.
 
-- ### Property setting rules
+- **Property setting rules**
 
     - If a table value is another table, that nested table is processed so that
       any properties inside that table are also applied to the instance just
       like the outer table.
     - If a table index is a string:
       - If its value is a function then it will either bind that property to
-        a state or connect it if the property type is a `RBXScriptSignal`.
+        the function or connect it if the property type is a `RBXScriptSignal`.
       - If the value is not a function then the property will be set to that
         value.
     - If a table index is a number:
-      - If its value is a function then it will parent any instances returned by
-        that function as children.
+      - If its value is a function then it will parent and bind any instances
+        returned by that function as children.
       - If its value is an instance then it will be parented to the instance.
 
-- ### Example
+- **Example**
 
     Basic element creation.
 
@@ -77,3 +77,44 @@ Creates a new UI element, applying any given properties.
         }
     end
     ```
+
+## action()
+
+Creates a callback that can be passed to `create()` to invoke custom actions on
+instances.
+
+- **Type**
+
+    ```lua
+    function action((Instance) -> (), priority: number = 1): Action
+    ```
+
+- **Details**
+
+    When passed to `create()`, the given callback is called with the instance
+    being created as the only argument. Actions take precedence over property
+    and child assignments.
+
+    A priority can be optionally specified to ensure certain actions run after
+    other actions. Higher priority numbers are ran after lower priority numbers.
+
+- **Example**
+
+    An action to listen to changed properties:
+
+    ```lua
+    local function changed(property: string, callback: (new) -> ())
+        return action(function(instance)
+            instance:GetPropertyChangedSignal("property"):Connect(function()
+                callback(instance[property])
+            end)
+        end)
+    end
+
+    local output = source ""
+
+    create "TextBox" {
+        -- will update the `output` source anytime the text property is changed
+        changed("Text", output)
+    }
+    ```