UG: Add help window known issue

The Help Window cannot be activated if the previous
Help Window is still alive but minimized.

Until there is a better solution, let's mention this problem
in the 'Known issues' section of the UG.

Author: hjungwoo01

docs/UserGuide.md

@@ -182,6 +182,7 @@ _Details coming soon ..._
 ## Known issues
 
 1. **When using multiple screens**, if you move the application to a secondary screen, and later switch to using only the primary screen, the GUI will open off-screen. The remedy is to delete the `preferences.json` file created by the application before running the application again.
+2. **If you minimize the Help Window** and then run the `help` command (or use the `Help` menu, or the keyboard shortcut `F1`) again, the original Help Window will remain minimized, and no new Help Window will appear. The remedy is to manually restore the minimized Help Window.
 
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