Rollup merge of #75454 - ltratt:option_optimisation_guarantees, r=dtolnay
Rollup merge of #75454 - ltratt:option_optimisation_guarantees, r=dtolnay

Explicitly document the size guarantees that Option makes.

Triggered by a discussion on wg-unsafe-code-guidelines about which layouts of `Option<T>` one can guarantee are optimised to a single pointer.

CC @RalfJung

graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust