Rollup merge of #53024 - matklad:patch-1, r=alexcrichton

Specify reentrancy gurantees of `Once::call_once`

I don't think the docs are clear about what happens in the following code

```rust
static INIT: Once = ONCE_INIT;

INIT.call_once(|| INIT.call_once(|| println!("huh?")));
```

[Playground](https://play.rust-lang.org/?gist=15dde1f68a6ede263c7250c36977eade&version=stable&mode=debug&edition=2015)

Let's "specify" the behavior to make it clear that the current behavior (deadlock I think?) is not a strict guarantee.

src/libstd/sync/once.rs

@@ -178,6 +178,10 @@ impl Once {
     /// happens-before relation between the closure and code executing after the
     /// return).
     ///
+    /// If the given closure recusively invokes `call_once` on the same `Once`
+    /// instance the exact behavior is not specified, allowed outcomes are
+    /// a panic or a deadlock.
+    ///
     /// # Examples
     ///
     /// ```