Rollup merge of rust-lang#116172 - joboet:recursive_tls_initializatio…

…n, r=dtolnay

Broaden the consequences of recursive TLS initialization

This PR updates the documentation of `LocalKey` to clearly disallow the behaviour described in [this comment](rust-lang#110897 (comment)). This allows using `OnceCell` for the lazy initialization of TLS variables, which panics on reentrant initialization instead of updating the value like TLS variables currently do.

`@rustbot` label +T-libs-api
r? `@m-ou-se`

library/std/src/thread/local.rs

@@ -29,9 +29,9 @@ use crate::fmt;
 /// within a thread, and values that implement [`Drop`] get destructed when a
 /// thread exits. Some caveats apply, which are explained below.
 ///
-/// A `LocalKey`'s initializer cannot recursively depend on itself, and using
-/// a `LocalKey` in this way will cause the initializer to infinitely recurse
-/// on the first call to `with`.
+/// A `LocalKey`'s initializer cannot recursively depend on itself. Using a
+/// `LocalKey` in this way may cause panics, aborts or infinite recursion on
+/// the first call to `with`.
 ///
 /// # Examples
 ///