Clarify initialized union may contain uninited bytes

[hehaoqian]

New to Rust, just realized this after resolving an issue related to this.

This can be verified by sample code: Rust Playground

Each execution gives different output

#[repr(C)]
union Foo {
    int32: i32,
    int64: i64,
}
unsafe {
    let foo = Foo { int32: 1 };
    // Low 4 bytes are initialized,
    // but high 4 bytes are not.
    println!("{}", foo.int64);
}

[traviscross]

Thanks @hehaoqian for proposing this. We're going to close this PR since it's explained further down in the page what the validity constraints are when using unions, and that seems sufficient.