Clarify timing of Area3D signals

**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