Add back docs for the old way of testing promises (#3256)
Add back docs for the old way of testing promises (#3256)

* Add back docs for the old way of testing promises

Fixes #3250

* Add `return` note to old version

ide/jest,bookman25/jest,mpontus/jest,bookman25/jest,cpojer/jest,skovhus/jest,bookman25/jest,mpontus/jest,facebook/jest,skovhus/jest,mpontus/jest,cpojer/jest,ide/jest,ide/jest,cpojer/jest,facebook/jest,facebook/jest,skovhus/jest