improve wording

Author: RalfJung

src/items/unions.md

@@ -39,13 +39,14 @@ let f = u.f1;
 
 Unions have no notion of an "active field". Instead, every union access just
 interprets the storage at the type of the field used for the access. Reading a
-union field is equivalent to a [`transmute`]: The data in the union, no matter
-how it was stored there, is transmuted to the type of the field. Just like with
-any other transmute, it is the programmer's responsibility to make sure that the
-data is valid at the field's type. Failing to do so results in undefined
-behavior (for example, reading the value `3` at type `bool` is undefined
-behavior). For this reason, all reads of union fields have to be placed in
-`unsafe` blocks:
+union field reads the bits of the union at the field's type. It is the
+programmer's responsibility to make sure that the data is valid at that
+type. Failing to do so results in undefined behavior (for example, reading the
+value `3` at type `bool` is undefined behavior). Effectively, writing to and
+then reading from a union is equivalent to a [`transmute`] from the type used
+for writing to the type used for reading.
+
+Consequently, all reads of union fields have to be placed in `unsafe` blocks:
 
 ```rust
 # union MyUnion { f1: u32, f2: f32 }