Clarify timing of Area3D signals

[pshaughn]

Your Godot version: 4.5

Issue description: Various signals are described as emitted "when" a shape enters/exits the area. This doesn't seem to be a good description of the actual behavior; for instance, a CharacterBody performing a bunch of move_and_collide steps during the same physics tick doesn't cause a body_entered signal to fire at the time of any one individual move. I don't have a suggested better wording because I'm not myself sure what the actual timing is.

URL to the documentation page (if already existing): https://docs.godotengine.org/en/stable/classes/class_area3d.html