[DOCS] rephrase impulse

 - improove coordinate system explanation
 - define usecase

doc/classes/PhysicsDirectBodyState.xml

@@ -57,7 +57,7 @@
 			<argument index="1" name="j" type="Vector3">
 			</argument>
 			<description>
-				Apply a positioned impulse (which will be affected by the body mass and shape). This is the equivalent of hitting a billiard ball with a cue: a force that is applied once, and only once. Both the impulse and the position are in global coordinates, and the position is relative to the object's origin.
+				Applies a positioned impulse to the body. An impulse is time independent! Applying an impulse every frame would result in a framerate dependent force. For this reason it should only be used when simulating one-time impacts. The position uses the rotation of the global coordinate system, but is centered at the object's origin.
 			</description>
 		</method>
 		<method name="apply_torque_impulse">

doc/classes/RigidBody.xml

@@ -72,7 +72,7 @@
 			<argument index="1" name="impulse" type="Vector3">
 			</argument>
 			<description>
-				Applies a positioned impulse which will be affected by the body mass and shape. This is the equivalent of hitting a billiard ball with a cue: a force that is applied once, and only once. Both the impulse and the position are in global coordinates, and the position is relative to the object's origin.
+				Applies a positioned impulse to the body. An impulse is time independent! Applying an impulse every frame would result in a framerate dependent force. For this reason it should only be used when simulating one-time impacts. The position uses the rotation of the global coordinate system, but is centered at the object's origin.
 			</description>
 		</method>
 		<method name="apply_torque_impulse">

doc/classes/RigidBody2D.xml

@@ -69,7 +69,7 @@
 			<argument index="1" name="impulse" type="Vector2">
 			</argument>
 			<description>
-				Applies a positioned impulse to the body (which will be affected by the body mass and shape). This is the equivalent of hitting a billiard ball with a cue: a force that is applied instantaneously. Both the impulse and the offset from the body origin are in global coordinates.
+				Applies a positioned impulse to the body. An impulse is time independent! Applying an impulse every frame would result in a framerate dependent force. For this reason it should only be used when simulating one-time impacts (use the "_force" functions otherwise). The position uses the rotation of the global coordinate system, but is centered at the object's origin.
 			</description>
 		</method>
 		<method name="apply_torque_impulse">