Clarify why reading and writing to union fields is unsafe

Author: carols10cents

_posts/2017-07-20-Rust-1.19.md

@@ -42,7 +42,9 @@ union MyUnion {
 Unions are kind of like `enum`s, but they are "untagged". Enums have a "tag"
 that stores which variant is the correct one at runtime; unions elide this tag.
 
-That means that reading or writing a union's field is unsafe:
+Since we can interpret the data held in the union using the wrong variant and
+Rust can't check this for us, that means reading or writing a union's field is
+unsafe:
 
 ```rust
 let u = MyUnion { f1: 1 };