There isn't but every shortcut should have a corresponding menu item and the menu item tells you if there is a shortcut for it.
So if you (for example) right click a downloaded subtitle, you'll see the shortcut next to the possible actions. And if you select 'Window' in the main top menu, you'll see that Cmd-1,2,3,4 switches to the different functions. etc.
The only undocumented one is that in many cases, you can use 'Shift' together with a shortcut. With Shift, it does the same action but for all items.
So Ctrl-i will move the selected video to Add to iTunes and Shift-Ctrl-i will move them all.