revised docs

zeroize/src/lib.rs

@@ -796,9 +796,10 @@ unsafe fn volatile_set<T: Copy + Sized>(dst: *mut T, src: T, count: usize) {
 }
 
 /// Zeroizes a flat type/struct. Only zeroizes the values that it owns, and it does not work on
-/// dynamically sized values or trait objects.
+/// dynamically sized values or trait objects. Do not use this function on a type that already
+/// implements `ZeroizeOnDrop`.
 ///
-/// ## Safety:
+/// # Safety:
 /// - The type must not contain references to outside data or dynamically sized data
 /// - This function can invalidate the type if it is used after this function is called on it. It is
 ///   advisable to call this method in `impl Drop`.