Rollup merge of #161245 - RalfJung:target-feature-required-for-vector, r=jieyouxu

document meaning of empty feature name in correct_fixed_length_vector_abi tables

In a9b147259be705fba32fa5dbf887b7c10eef701d we apparently added a hack where an empty feature name means "no target feature required". An `Option` may have been a cleaner way of doing this, but meanwhile, let's at least document this hack.