Clarify the guarantees that ThreadId does and doesn't make.

library/std/src/thread/mod.rs

@@ -965,10 +965,13 @@ pub fn park_timeout(dur: Duration) {
 
 /// A unique identifier for a running thread.
 ///
-/// A `ThreadId` is an opaque object that has a unique value for each thread
-/// that creates one. `ThreadId`s are not guaranteed to correspond to a thread's
-/// system-designated identifier. A `ThreadId` can be retrieved from the [`id`]
-/// method on a [`Thread`].
+/// A `ThreadId` is an opaque object that uniquely identifies each thread
+/// created during the lifetime of a process. `ThreadId`s are guaranteed not to
+/// be reused, even when a thread terminates. `ThreadId`s are under the control
+/// of Rust's standard library and there may not be any relationship between
+/// `ThreadId` and the underlying platform's notion of a thread identifier --
+/// the two concepts cannot, therefore, be used interchangeably. A `ThreadId`
+/// can be retrieved from the [`id`] method on a [`Thread`].
 ///
 /// # Examples
 ///