Docs updates and thread_id crate

tests/allocations.rs

@@ -86,4 +86,12 @@ fn test_vec_new() {
 fn test_vec_with_one() {
     enter_protected();
     let _v: Vec<u8> = Vec::with_capacity(1);
+}
+
+#[test]
+#[should_panic]
+fn exit_too_often() {
+    enter_protected();
+    exit_protected();
+    exit_protected();
 }