Change NOTIFICATION constants to suggests alternatives, and explain when they run

**Your Godot version:**
4.2 steam

**Issue description:**
While looking for a signal or notification nodes have that run after the `_ready()` function, I found the notifications, and assumed `NOTIFICATION_READY` ran after the `_ready()` function, considering its description of
> Notification received when the node is ready. See _ready().

After testing, my assumption did not appear to be the case. Additionally, in the linked `_ready()` function description, there is this:
> Corresponds to the NOTIFICATION_READY notification in Object._notification(). See also the @ onready annotation for variables.

I realize now that this more shows that the notification is sent either when `_ready()` runs or just before it runs, but it could still use some enhancement, such as linking to the `ready` signal, perhaps something along the lines of "For scripts that need to run after the `_ready()` function of another node, await the `ready` signal." Seeing as another constant refers to the related signal, stating that it runs before the signal, I believe that the other constants should be checked and given a link to their related signal, if they have one.

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

I do understand that I may be misunderstanding how these constants work, however in this case, from what I can tell, they run like signals being sent to the node itself.