Store#findHasMany return value and documentation fix

The documentation for `Store#findHasMany` was incorrectly changed in https://github.com/emberjs/data/commit/7ab338bea001437002b05a1965367e3e94c404c8.

I then realized that the intended behavior didn't work anyway so fixed that by returning the retrieved records from `_findHasMany`.
this should be a promise