UndoRedo documentation lacks information about 800ms interval, and needs clarfication on merging

**Your Godot version:** 4.2.rc

**Issue description:**

UndoRedo documentation is lacking information about how merging of actions work. It only gives a brief overview of MERGE_DISABLE, MERGE_ENDS and MERGE_ALL and doesn't give any code examples on how their behaviour differs. In order to fully understand intended behaviour, I made my own chart:


Also of note is that the documentation makes no mention that actions have to take place within 800ms of eachother in order to be merged. I've seen multiple people being surprised by this behavior, including code contributors.

**URL to the documentation page:** https://docs.godotengine.org/en/stable/classes/class_undoredo.html