QtIFW

Fix CreateLink operation destination string formatting on Windows

- Fix string formatting syntax in createJunction() method.
- Calculate destination directory string length in bytes rather than
  characters count.
- Make minor alterations to operation usage documentation.

Task-number: QTIFW-1443
Change-Id: Ia7f6525c865a3e8a6da5376059111c1ef87353ac
Reviewed-by: Katja Marttila <katja.marttila@qt.io>