Update documentation on optimistic update
Update documentation on optimistic update

Summary:
Closes #2365.
Closes https://github.com/facebook/relay/pull/2366

Differential Revision: D7377051

Pulled By: kassens

fbshipit-source-id: dabed681cb9b2834ec8d3adeef0e55b9a74feaab

zpao/relay,josephsavona/relay,xuorig/relay,xuorig/relay,voideanvalue/relay,facebook/relay,wincent/relay,yungsters/relay,cpojer/relay,wincent/relay,voideanvalue/relay,voideanvalue/relay,iamchenxin/relay,dbslone/relay,voideanvalue/relay,iamchenxin/relay,facebook/relay,wincent/relay,xuorig/relay,wincent/relay,yungsters/relay,facebook/relay,xuorig/relay,iamchenxin/relay,kassens/relay,josephsavona/relay,zpao/relay,facebook/relay,atxwebs/relay,atxwebs/relay,cpojer/relay,cpojer/relay,freiksenet/relay,dbslone/relay,cpojer/relay,freiksenet/relay,iamchenxin/relay,wincent/relay,kassens/relay,voideanvalue/relay,kassens/relay,atxwebs/relay,voideanvalue/relay,zpao/relay,kassens/relay,facebook/relay,wincent/relay,josephsavona/relay,kassens/relay,yungsters/relay,atxwebs/relay,kassens/relay,xuorig/relay,facebook/relay,xuorig/relay,freiksenet/relay