[2.1.7] `LuaUndoRedoStack.get_*_item_count()` improperly removed from 2.1 docs

[The_LORD_thy_GOD]

`get_undo_item_count()` and `get_redo_item_count()` (fortunately) still exist in the game but appear to have incorrectly been removed from the docs for 2.1.7

[Therenas]

Thanks for the report. A few dozen methods got dropped in fact, unfortunately. Sorry about that. Fixed for the next release.