docs: add a note about response body

docs/fetching/choosing.md

@@ -77,4 +77,8 @@ The `Response` object is the same as the [Selector](../parsing/main_classes.md#s
 >>> page.encoding        # Response encoding
 >>> page.meta            # Response metadata dictionary (e.g., proxy used). Mainly helpful with the spiders system.
 ```
-All fetchers return the `Response` object.
+All fetchers return the `Response` object.
+
+!!! note
+
+    Unlike the [Selector](../parsing/main_classes.md#selector) class, the `Response` class's body is always bytes since v0.4.