Issues #22468, #21996, #22208: Clarify gettarinfo() and TarInfo usage

* The Windows-specific binary notice was probably a Python 2 thing
* Make it more obvious gettarinfo() is based on stat(), and that non-ordinary
  files may need special care
* The file name must be text; suggest dummy arcname as a workaround
* Indicate TarInfo may be used directly, not just via gettarinfo()