Added clarification for npm3 and unmocking
Added clarification for npm3 and unmocking

Summary: Closes https://github.com/facebook/jest/pull/604

Reviewed By: svcscm

Differential Revision: D2647840

fb-gh-sync-id: a8c9ab5ed9cc4f3356916d9049b86bd6f5763238

sampepose/jest,mpontus/jest,aaron-goshine/jest,skovhus/jest,blainekasten/jest,cpojer/jest,ide/jest,razh/jest,skawian/jest,cpojer/jest,bookman25/jest,blainekasten/jest,yungsters/jest,razh/jest,facebook/jest,skawian/jest,skovhus/jest,bookman25/jest,mpontus/jest,aaron-goshine/jest,skovhus/jest,facebook/jest,Daniel15/jest,ide/jest,alvinsj/jest,cpojer/jest,aaron-goshine/jest,mpontus/jest,ncuillery/jest,Daniel15/jest,ide/jest,bookman25/jest,sampepose/jest,ncuillery/jest,yungsters/jest,Daniel15/jest,facebook/jest,alvinsj/jest,blainekasten/jest