feat(p3): only allow body.stream calls once

wit-0.3.0-draft/types.wit

@@ -265,8 +265,7 @@ interface types {
 
     /// Returns the contents of the body, as a stream of bytes.
     ///
-    /// This function may be called multiple times as long as any `stream`s
-    /// returned by previous calls have been dropped first.
+    /// This can be called at most once per body.
     ///
     /// On success, this function returns a stream and a future, which will resolve
     /// to an error code if receiving data from stream fails.