Update comment

Signed-off-by: Xuanwo <github@xuanwo.io>

Author: Xuanwo

src/common/base/src/base/singleton_instance.rs

@@ -129,6 +129,7 @@ impl GlobalInstance {
     /// Should only be used in testing code.
     #[cfg(debug_assertions)]
     pub fn drop_testing(thread_name: &str) {
+        // Make sure the write lock is released before alling drop.
         let _ = { LOCAL.wait().write().remove(thread_name) };
     }