update and extend some comments, and cfg-out some unused code

library/std/src/thread/local.rs

@@ -1036,6 +1036,7 @@ pub mod fast {
 }
 
 #[doc(hidden)]
+#[cfg(not(target_thread_local))]
 pub mod os {
     use super::lazy::LazyKeyInner;
     use crate::cell::Cell;
@@ -1044,6 +1045,8 @@ pub mod os {
     use crate::ptr;
     use crate::sys_common::thread_local_key::StaticKey as OsStaticKey;
 
+    /// Use a regular global static to store this key; the state provided will then be
+    /// thread-local.
     pub struct Key<T> {
         // OS-TLS key that we'll use to key off.
         os: OsStaticKey,