docs: `transmute<&mut T, &mut MaybeUninit<T>>` is unsound when exposed to safe code
Closes#66699
On my system (Edit: And also in the [playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=90529e2a9900599cb759e4bfaa5b5efe)) the example program terminates with an unpredictable exit code:
```console
$ cargo +nightly build && target/debug/bin ; echo $?
255
$ cargo +nightly build && target/debug/bin ; echo $?
253
```
And miri considers the code to have undefined behavior:
```console
$ cargo +nightly miri run
error: Undefined Behavior: using uninitialized data, but this operation requires initialized memory
--> src/main.rs:12:24
|
12 | std::process::exit(*code); // UB! Accessing uninitialized memory
| ^^^^^ using uninitialized data, but this operation requires initialized memory
|
error: aborting due to 1 previous error
```
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