There was some confusion about the correct way to add an `All Files` filter (see #2525), so I added it to the example.
Also added a short note about the extension syntax.
This is related to PR #2547.
Added a comment about the fact that on Windows, the "question" message box uses the same icon as the "info" message box (Electron uses the task dialog API instead of the message box API). This is because Microsoft's API doesn't supply a default question icon (see `pszIcon` in https://msdn.microsoft.com/en-us/library/windows/desktop/bb760540(v=vs.85).aspx).
