Remove definition and rationale

Author: gnzlbg

reference/src/validity/unions.md

@@ -4,22 +4,10 @@
 represents the consensus from issue [#73]. The statements in here are not (yet)
 "guaranteed" not to change until an RFC ratifies them.
 
-The bit `i` of an union is allowed to have value `v` _iff_ there is a variant of
-the union such that bit `i` of the variant is allowed to have value `v`. We
-assume all variants to be "filled up" to the same size with padding, which may
-have any value.
-
 ## Validity of unions with zero-sized fields
 
 A union containing a zero-sized field can contain any value. An example of such
 an union is [`MaybeUninit`].
 
-<details><summary><b>Rationale</b></summary> 
-
-This follows from the definition of the values that `union` bits are allowed to
-take. The zero-sized type has size `0`, and its variant is filled up to
-the `union` size with padding. Since padding is allowed to take any value, any
-bit in the `union` is allowed to take any value.
-
 [#73]: https://github.com/rust-lang/unsafe-code-guidelines/issues/73
 [`MaybeUninit`]: https://doc.rust-lang.org/std/mem/union.MaybeUninit.html