Clarify CanvasItems Y-sorting functionality

**Your Godot version:**
4.7.1

**Issue description:**
The description of Y-Sorting is potentially misleading.

> "If true, ***this*** and child `CanvasItem` nodes with a higher Y position are rendered in front of nodes with a lower Y position."

This can be confusing because it might suggest that nodes will Y-sort with other sibling nodes as long as both siblings have Y-sort enabled. This is because the description says **THIS** node will Y-sort when it's enabled. While this is true, it only will Y-sort with its own children, not siblings or anything else, and that's a little unclear in my opinion.

Just add a sentence like "When a node has Y-sorting enabled, Y-sorting will only necessarily occur among the node itself and all of its direct children."

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/classes/class_canvasitem.html#class-canvasitem-property-y-sort-enabled

Start with the CanvasItem class reference page at the linked Y-sort-enabled property. Clarify that Y-sorting does not necessarily apply between sibling nodes, then verify the rendered documentation explains the sorting scope without suggesting that independently enabled siblings will sort together.

Written by the indexing model from the issue text.