Add clarification about when instance_vars can be called

src/compiler/crystal/macros.cr

@@ -1752,6 +1752,7 @@ module Crystal::Macros
     end
 
     # Returns the instance variables of this type.
+    # Can only be called from methods, it will return an empty list if called from top-level code.
     def instance_vars : ArrayLiteral(MetaVar)
     end