note down new APIs that have been added after we formalized this

Missing.md

+Missing APIs from the types we cover (APIs have been added after this formalization was done)
+
+# Cell
+
+* Structural conversion for slices.  The matching operations in our model would be
+  `&mut Cell<(A, B)>` -> `(&mut Cell<A>, &mut Cell<B>)` and
+  `&Cell<(A, B)>` -> `(&Cell<A>, &Cell<B>)`.
+
+# RefCell
+
+* RefMut::split: https://github.com/rust-lang/rust/pull/51466