Rollup merge of #130653 - RalfJung:result-abi-compat, r=traviscross

ABI compatibility: mention Result guarantee

This has been already documented in https://doc.rust-lang.org/std/result/index.html#representation, but for `Option` we mirrored those docs in the "ABI compatibility" section, so let's do the same here.

Cc ``@workingjubilee`` ``@rust-lang/lang``

Author: compiler-errors