Add documentation guaranteeing global allocator use of TLS

Author: orlp

library/std/src/thread/local.rs

@@ -24,7 +24,13 @@ use crate::fmt;
 /// [`with`]) within a thread, and values that implement [`Drop`] get
 /// destructed when a thread exits. Some platform-specific caveats apply, which
 /// are explained below.
-/// Note that, should the destructor panics, the whole process will be [aborted].
+/// Note that, should the destructor panic, the whole process will be [aborted].
+/// On platforms where initialization requires memory allocation, this is
+/// performed directly through [`System`], allowing the [`global allocator`]
+/// to make use of thread local storage.
+///
+/// [`System`]: crate::alloc::System
+/// [`global allocator`]: crate::alloc
 ///
 /// A `LocalKey`'s initializer cannot recursively depend on itself. Using a
 /// `LocalKey` in this way may cause panics, aborts or infinite recursion on