Improve header comment in test file.

In particular, report that the encoding of the
test file is UTF-16 (Little Endian), because the
test file includes some non-Latin characters.

Notes:

- AS Editor usually saves files with the ASCII or MacOS Roman
  encoding (even in Mavericks!). When a file contains
  non-Latin characters, then the file is automatically
  saved with UTF-16LE encoding (if you remove those
  characters and save the file again, AS Editor switches
  to MacOS Roman or ASCII again). I don't think AS Editor ever
  uses UTF-8 to save files.
- As of today, AppleScriptObjC Explorer 3 does not
  handle UTF-16LE, but Script Debugger 4.5 does.
- Script Debugger saves files in UTF8, with BOM.
- AppleScriptObjC Explorer 3 saves files in UTF (no BOM).
- HeaderDoc 8.9 does not seem to support UTF-16LE.
  If you pass a UTF-16LE-encoded file to headerdoc2html,
  it won't output anything.
- Btw, Terminal.app does not work with UTF-16LE either,
  so you cannot diff such files.

In summary, a total mess.