Make the intent clearer in manage_as_static

Summary: Add a comment clarifying why we use a ctor that alliases an empty `shared_ptr` in `manage_as_static`. Without a comment it's a bit hard to figure out the behavior we get from the docs in https://en.cppreference.com/w/cpp/memory/shared_ptr/shared_ptr, esp. since the set of relevant overloads changed in C++20.

Reviewed By: praihan

Differential Revision: D70422993

fbshipit-source-id: de642aa1f4d7ba73c82821923a052b666c54b3f6

thrift/compiler/whisker/managed_ptr.h

@@ -44,6 +44,8 @@ using managed_ptr = std::shared_ptr<const T>;
  */
 template <typename T>
 managed_ptr<T> manage_as_static(const T& o) {
+  // Alias an empty shared_ptr to create a non-owning shared_ptr that doesn't
+  // allocate a control block (unlike using a noop deleter).
   return managed_ptr<T>(managed_ptr<void>(), std::addressof(o));
 }