doc: Clarifying NavigationAgent and NavigationAgent2D velocity_computed requires avoidance_enabled to be true to emit

(cherry picked from commit ed5b832a81bb7f67924282d2c3f96b8674ecbb7d)

doc/classes/NavigationAgent.xml

@@ -169,7 +169,7 @@
 		<signal name="velocity_computed">
 			<argument index="0" name="safe_velocity" type="Vector3" />
 			<description>
-				Notifies when the collision avoidance velocity is calculated after a call to [method set_velocity].
+				Notifies when the collision avoidance velocity is calculated after a call to [method set_velocity]. Only emitted when [member avoidance_enabled] is true.
 			</description>
 		</signal>
 	</signals>

doc/classes/NavigationAgent2D.xml

@@ -163,7 +163,7 @@
 		<signal name="velocity_computed">
 			<argument index="0" name="safe_velocity" type="Vector2" />
 			<description>
-				Notifies when the collision avoidance velocity is calculated after a call to [method set_velocity].
+				Notifies when the collision avoidance velocity is calculated after a call to [method set_velocity]. Only emitted when [member avoidance_enabled] is true.
 			</description>
 		</signal>
 	</signals>