Clarify comment explaining how referenced works

include/asm/fstack.hpp

@@ -20,7 +20,7 @@ struct FileStackNode {
 	// Line at which the parent context was exited; meaningless for the root level
 	uint32_t lineNo;
 
-	bool referenced; // If referenced, don't free!
+	bool referenced; // If referenced by a Symbol, Section, or Patch's `src`, don't `delete`!
 	uint32_t ID; // Set only if referenced: ID within the object file, -1 if not output yet
 
 	enum FileStackNodeType type;