Rollup merge of #138000 - RalfJung:atomic-rmw, r=Amanieu

atomic: clarify that failing conditional RMW operations are not 'writes'

Fixes rust-lang/rust#136669

r? ``@Amanieu``
Cc ``@rust-lang/opsem`` ``@chorman0773`` ``@gnzlbg`` ``@briansmith``

Author: jhpratt