Clarify that drop flag fields only apply to older Rust versions (#324)

Co-authored-by: Yuki Okushi <jtitor@2k36.org>

src/drop-flags.md

@@ -79,5 +79,5 @@ if condition {
 }
 ```
 
-The drop flags are tracked on the stack and no longer stashed in types that
-implement drop.
+The drop flags are tracked on the stack.
+In old Rust versions, drop flags were stashed in a hidden field of types that implement `Drop`.