The shortcuts for Forward/Backward in Document History are Ctrl+[ and Ctrl+]. They are not well documented at all–I don’t think this function exists in the menus; it does show up in Tools > Options > Keyboard, near the end of the View section.
Glad it helped! Note that in that Tools > Options > Keyboard dialog, it’s possible to change any of the keyboard shortcut assignments to something you like better. I keep thinking I want to change that one to Alt+Right Arrow/Left Arrow, as in browsers and some other programs, but haven’t gotten around to it yet. (By default, Alt+Right Arrow/Left Arrow shows and hides the editing toolbar.)