Update Directives.md
Update Directives.md

Summary:
Clarify that these are client-side directives and aren't sent to the server and support all server-side directives
Closes https://github.com/facebook/relay/pull/2112

Differential Revision: D5874949

Pulled By: leebyron

fbshipit-source-id: 3196ba5e24fb9f264e9e0b43e346058c307cf575

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