Clarify Shapecast safe/unsafe fraction difference

doc/classes/ShapeCast2D.xml

@@ -48,6 +48,7 @@
 			<return type="float" />
 			<description>
 				The fraction from the [ShapeCast2D]'s origin to its [member target_position] (between 0 and 1) of how far the shape must move to trigger a collision.
+				In ideal conditions this would be the same as [method get_closest_collision_safe_fraction], however shape casting is calculated in discrete steps, so the precise point of collision can occur between two calculated positions.
 			</description>
 		</method>
 		<method name="get_collider" qualifiers="const">

doc/classes/ShapeCast3D.xml

@@ -48,6 +48,7 @@
 			<return type="float" />
 			<description>
 				The fraction from the [ShapeCast3D]'s origin to its [member target_position] (between 0 and 1) of how far the shape must move to trigger a collision.
+				In ideal conditions this would be the same as [method get_closest_collision_safe_fraction], however shape casting is calculated in discrete steps, so the precise point of collision can occur between two calculated positions.
 			</description>
 		</method>
 		<method name="get_collider" qualifiers="const">