assocs: improve stack effect docs for "?at".
assocs: improve stack effect docs for "?at".

erg/factor,mcandre/factor,erg/factor,mcandre/factor,mcandre/factor,erg/factor,mcandre/factor,erg/factor,erg/factor,mcandre/factor,mcandre/factor,erg/factor,mcandre/factor,erg/factor