UndoRedo class page: more elaborate method descriptions and better example code

**Your Godot version:** 3.4.5

**Issue description:** The documentation for the UndoRedo class is noticeably sparse, with vague method descriptions (see the [add_do_property()](https://docs.godotengine.org/en/stable/classes/class_undoredo.html#class-undoredo-method-add-do-property) method for an example) and example code that does literally nothing perceptible.

For someone trying to learn to use UndoRedo for the first time, there are simply too many blanks.

Given the importance and potential complexity of an undo history, I believe there is a strong need to deepen the information available and, more specifically, add sections explaining how to introduce the construction, **_modification,_** and deletion of objects into the undo history.

**URL to the documentation page (if already existing):** https://docs.godotengine.org/en/stable/classes/class_undoredo.html

Thank you.