Y-sorting is an incredibly important tool in non-tile-based 2D games, and I only truly understood Godot's robust implementation of it after wasting 2 days writing complex scripts to control Sprite offsets as I did not comprehend the description for y_sort_enabled. This PR aims to improve that description so that I can save others the experimentation.

In short:

• I merged the two first sentences as they were saying the same thing twice.
• I attempted to actually explain what disabling y-sorting does instead of stating the obvious 'if you disable this, you disable this.'
• I rewrote the second part to clarify that disabling y-sorting essentially groups the nodes under the y position of the parent if the outermost parent is Y-sorted. This concept is in my opinion quite complex and I think the previous description failed to describe the affordance of that complexity, namely: you can turn y_sort_enabled on and off on selected nodes to group and Y-sort the entire scene in a smart way.
• I added that Y-sorting only works if those nodes are on the same Z-index. I was a bit in doubt about adding this, as it makes the whole tooltip another two lines higher, but decided to add it because there is a relationship with Z-index that isn't explained anywhere else.

As a bonus, I also removed the redundant 'Z index' at the start of the description for z_index. If that should be a separate PR, let me know.