…g, r=oli-obk

Clarify `invalid_reference_casting` lint around interior mutable types

This is PR intends to clarify the `invalid_reference_casting` lint around interior mutable types by adding a note for them saying that they should go through `UnsafeCell::get`.

So for this code:
```rust
let cell = &std::cell::UnsafeCell::new(0);
let _num = &mut *(cell as *const _ as *mut i32);
```

the following note will be added to the lint output:

```diff
 error: casting `&T` to `&mut T` is undefined behavior, even if the reference is unused,  consider instead using an `UnsafeCell`
   --> $DIR/reference_casting.rs:68:16
    |
 LL |     let _num = &mut *(cell as *const _ as *mut i32);
    |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: for more information, visit <https://doc.rust-lang.org/book/ch15-05-interior-mutability.html>
+   = note: even for types with interior mutability, the only legal way to obtain a mutable pointer from a shared reference is through `UnsafeCell::get`
```

Suggestion are welcome around the note contents.

Fixes rust-lang#116410
cc `@RalfJung`

…jubilee

Fix typo in README.md

 the word "programatically" should be corrected to "programmatically."