add comment to help people debugging an assertion failure

Summary:
SharedMutex asserts that all locks have been released before
destruction.  This diff adds a comment near those asserts making it more
clear what they are checking.

Reviewed By: yfeldblum

Differential Revision: D7137906

fbshipit-source-id: 082a804616950d9a962ee932c9021c453f43a35d

folly/SharedMutex.h

@@ -274,6 +274,10 @@ class SharedMutexImpl {
     }
 
 #ifndef NDEBUG
+    // These asserts check that everybody has released the lock before it
+    // is destroyed.  If you arrive here while debugging that is likely
+    // the problem.  (You could also have general heap corruption.)
+
     // if a futexWait fails to go to sleep because the value has been
     // changed, we don't necessarily clean up the wait bits, so it is
     // possible they will be set here in a correct system