thread_loca::guard: make the enable contract more explicit

Author: RalfJung

library/std/src/sys/thread_local/guard/windows.rs

@@ -106,6 +106,7 @@ impl Drop for EnableGuard {
     }
 }
 
+/// Set up the current thread to invoke `cleanup` when it finishes.
 pub fn enable() {
     let registered = if cfg!(target_thread_local) {
         #[thread_local]

library/std/src/sys/thread_local/mod.rs

@@ -81,7 +81,7 @@ pub(crate) mod destructors {
 
 /// This module provides a way to schedule the execution of the destructor list
 /// and the [runtime cleanup](crate::rt::thread_cleanup) function. Calling `enable`
-/// should ensure that these functions are called at the right times.
+/// sets up the current thread to ensure that these functions are called at the right times.
 pub(crate) mod guard {
     cfg_select! {
         all(target_thread_local, target_vendor = "apple") => {