message: Clarify behaviour in README for closing a File
diff: - Errors are not expanded when encountered, but only when being used for printing or by
  the library user.
  - Handles to `hdf5` identifiers are no longer tracked via a registry and is instead handled by stricter semantics of ownership.
  +- The handle to a `File` will not close all objects in a `File` when dropped, but instead uses a weak file close degree. For the old behaviour see `FileCloseDegree::Strong`.
  ### Fixed
  