Address review comment.

Author: crlf0710

reference/src/layout/pointers.md

@@ -51,6 +51,8 @@ struct DynObject {
 }
 ```
 
+> **note**: In the layout equivalence of `&dyn mut Trait` the field `data` is of the type `*mut u8`.
+
 The layout of `&[T]` is the same as that of:
 ```rust
 #[repr(C)]
@@ -60,4 +62,7 @@ struct Slice<T> {
 }
 ```
 
-The layout of `&str` is the same as that of `&[u8]`.
+> **note**: In the layout equivalence of `&mut [T]` the field `ptr` is of the type `*mut T`.
+
+The layout of `&str` is the same as that of `&[u8]`, and the layout of `&mut str` is
+the same as that of `&mut [u8]`.