cmStateTypes: document that the ordering of targets is important

The ordering is assumed in many places throughout the code and
modification of the enumeration should be done carefully.

Source/cmStateTypes.h

@@ -27,6 +27,9 @@ enum SnapshotType
   VariableScopeType
 };
 
+// There are multiple overlapping ranges represented here. Be aware that adding
+// a value to this enumeration may cause failures in numerous places which
+// assume details about the ordering.
 enum TargetType
 {
   EXECUTABLE,