gpui: Update docs to reflect removal of View, ViewContext, WindowContext (#24008)

This PR updates function signatures, docstrings, and gpui's other
documentation to reflect it's new state following the merge of `Model`
and `View` into `Entity` as well as the removal of `WindowContext`.

Release Notes:

- N/A

crates/gpui/src/subscription.rs

@@ -163,7 +163,7 @@ impl Subscription {
     }
 
     /// Detaches the subscription from this handle. The callback will
-    /// continue to be invoked until the views or models it has been
+    /// continue to be invoked until the entities it has been
     /// subscribed to are dropped
     pub fn detach(mut self) {
         self.unsubscribe.take();