sync: document sync.thread_id()

vlib/sync/thread_default.c.v

@@ -2,6 +2,14 @@ module sync
 
 fn C.pthread_self() usize
 
+// thread_id returns a unique identifier for the caller thread.
+// All *currently* running threads in the same process, will have *different* thread identifiers.
+// Note: if a thread finishes, and another starts, the identifier of the old thread may be
+// reused for the newly started thread.
+// In other words, thread IDs are guaranteed to be unique only within a process.
+// A thread ID may be reused after a terminated thread has been joined (with `t.wait()`),
+// or when the thread has terminated.
+
 pub fn thread_id() u64 {
 	return u64(C.pthread_self())
 }