Rollup merge of #132003 - RalfJung:abi-compat-docs, r=traviscross

update ABI compatibility docs for new option-like rules

Documents the rules decided [here](rust-lang/rust#130628 (comment)) for our ABI compatibility rules.

Long-term this should be moved to the reference, but for now this is what we got.

Cc `@rust-lang/lang` `@rust-lang/opsem`

Author: matthiaskrgr