Command - Option - Shift - / (aka Command - Option - ? ) Will open the documentation window.
In addition, you can switch between windows inside the application using Command - ` (works in any application). You can use this to easily switch between the project window and other open windows, including the documentation window.
Command - Tab Loops between applications, not windows.
Please note that the key combination for this menu command will be displayed in the menu along with the command (this is true for any application):

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