Add details about types with interior mutability to E0386.

Types with interior mutability like `Cell` and `RefCell` can be used to
skirt the restriction on mutating mutable values inside an immutable
container.

Author: nathankleyn

src/librustc_borrowck/diagnostics.rs

@@ -179,6 +179,14 @@ let mut x: i64 = 1;
 let mut y: Box<_> = Box::new(&mut x);
 **y = 2;
 ```
+
+It can also be fixed by using a type with interior mutability, such as `Cell` or
+`RefCell`:
+
+```
+let y: Cell<_> = Cell::new(1);
+y.set(2);
+```
 "##,
 
 E0387: r##"