Add a comment explaining exactly what is being tested here.

regress/usr.bin/config/SHADOW_INSTANCE

@@ -9,6 +9,16 @@ master0 	at root
 parenti* 	at master?
 parentii* 	at master?
 
+# The second child* instance (attaching at parentii) is useless: it will be
+# shadowed by the first one, which is semantically the same, from a parentii
+# device's point of view.
+#
+# The two child* instances are aliases, and at some point the orphan-checking
+# code skipped some aliases, in the shadowing situation and some others.
+#
+# This test should pass, the lines are valid, even though the second one is
+# useless.
+
 child* 	at hook?
 child* 	at parentii?