Clarify velocity_computed signal description

[rvenson]

As stated in the issue #81761, the velocity_computed signal is no longer triggered by NavigationAgent.set_velocity(). I updated the documentation for NavigationAgent2D and NavigationAgent3D to describe it cleaner since the current description could be misleading.

Current description:

Notifies when the collision avoidance velocity is calculated. Emitted when velocity is set. Only emitted when avoidance_enabled is true.

Updated description:

Notifies when the collision avoidance velocity is calculated. Emitted every update as long avoidance_enabled is true and the agent has a navigation map.

[Scony]

I'd change the wording from as long to as long as or from as long to whenever both

[rvenson]

I'd change the wording from as long to as long as or from as long to whenever both

Fixed. Thank you.

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉