doc: minor update to include global functions

Signed-off-by: Neo Xu <neo.xu1990@gmail.com>

src/lvgl.lua

@@ -332,9 +332,11 @@ end
 --- @param parent? Object | nil
 --- @param property? StyleProp
 --- @return Object
-function lvgl.Object(parent, property)
+function Object(parent, property)
 end
 
+lvgl.Object = Object
+
 --- Create Calendar widget on parent
 --- @param parent? Object | nil
 --- @param property? StyleProp
@@ -364,17 +366,21 @@ end
 --- @param parent? Object | nil
 --- @param property? ImageStyle
 --- @return Image
-function lvgl.Image(parent, property)
+function Image(parent, property)
 end
 
+lvgl.Image = Image
+
 ---
 --- Create Label on parent
 --- @param parent? Object | nil
 --- @param property? LabelStyle
 --- @return Label
-function lvgl.Label(parent, property)
+function Label(parent, property)
 end
 
+lvgl.Label = Label
+
 ---
 --- Create Textarea Widget on parent
 --- @param parent? Object | nil