Say only what a tail coupler does: end signals or none

The simulator reads the last vehicle's coupler for one thing, whether to put
up the end signals. It does not couple the train to anything behind it, as
the tooltip and a comment claimed.

StarterNG/Domain/Consist.cs

@@ -638,7 +638,7 @@ public sealed class Consist : IReadOnlyList<ConsistItem>
     /// <summary>
     /// Clears the coupler at the back of the consist, unless it was set in the
     /// editor. The simulator hangs the end signals only on a last vehicle whose
-    /// coupler is 0 - anything else couples it to whatever stands behind - and every
+    /// coupler is 0 - the code does nothing else at the back of a train - and every
     /// new car comes with 3. Its parameters, the brake setting among them, stay.
     /// </summary>
     private void FreeTail()