chore: document where the fields come from

juju/application.py

@@ -26,6 +26,16 @@
 
 
 class Application(model.ModelEntity):
+    """Represents the current state of a deployed application.
+
+    In the current library version, as well entire 2.x and 3.x series,
+    the data is supplied by Juju AllWatcher notifications, also known as deltas
+    for the specific application. The fields are declared here:
+    https://github.com/juju/juju/blob/be8a779/core/multiwatcher/types.go#L191-L209
+
+    The fields marked deprecated below will be removed in version 4.0 because
+    a different API must be used against Juju 4.
+    """
     @property
     def name(self) -> str:
         return self.entity_id