libs: Note lack of concurrency support (see #238)

Author: RyanGlScott

libs/std/src/sys/sync/thread_parking/mod.rs

@@ -1,2 +1,10 @@
+// For crucible-mir's benefit, we always use unsupported::Parker, which
+// implements all thread-parking functions as no-ops. This is arguably too
+// simplistic, as this skips calling operations that could trigger a context
+// switch in concurrent programs. Although crux-mir does have some support for
+// modeling this sort of concurrency, it has since bitrotted. (See #238.) If
+// this concurrency support is revived, then we will need to revisit the
+// approach here.
+
 mod unsupported;
 pub use unsupported::Parker;