rust: pin-init: improve documentation for `Zeroable` derive macros

Specify that both `MaybeZeroable` and `Zeroable` work on `union`s. Add a
doc example for a union. Also include an example with visibility on the
field.

Link: https://github.com/Rust-for-Linux/pin-init/pull/48/commits/ab0985a0e08df06c60a32ca5888f74adcc2c1cf3


Reviewed-by: Christian Schrefl <chrisi.schrefl@gmail.com>
Signed-off-by: Benno Lossin <benno.lossin@proton.me>