message: Add warning to LookupOrCreate about reentrancy issue
diff: @@ -131,6 +131,10 @@ class ResourceMgr {
  // "*resource". Otherwise, invokes creator() to create the resource.
  // The caller takes the ownership of one ref on "*resource".
  //
  + // WARNING: creator() must not call any methods on ResourceMgr during its
  + // execution, because a non-reentrant lock is held during the creator() call
  + // in order to guarantee atomicity of LookupOrCreate().
  + //
  // REQUIRES: std::is_base_of<ResourceBase, T>
  // REQUIRES: resource != nullptr
  template <typename T>
  