message: Update the documentation such that in one call of Registry::RegisterKeyManager we use a unique_ptr instead of a raw pointer.
diff: @@ -57,7 +57,8 @@ The registration of custom key managers can proceed directly via
  #include "custom_project/custom_aead_key_manager.h"
  // ...
  - auto status = Registry::RegisterKeyManager(new CustomAeadKeyManager());
  + auto status =
  + Registry::RegisterKeyManager(absl::make_unique<CustomAeadKeyManager>());
  if (!status.ok()) /* ... handle failure */;
  ```
  