[docs] Add docs for $ElementType<T, K> utility type
[docs] Add docs for $ElementType<T, K> utility type

Summary: Add docs for $ElementType<T, K> utility type

Reviewed By: calebmer

Differential Revision: D6336700

fbshipit-source-id: af3da87462e42564a398d2873fdadb5e3fa7547b

popham/flow,nmote/flow,JonathanUsername/flow,mroch/flow,mroch/flow,gabelevi/flow,popham/flow,claudiopro/flow,claudiopro/flow,AgentME/flow,TiddoLangerak/flow,jamesgpearce/flow,nmote/flow,JonathanUsername/flow,ylu1317/flow,ylu1317/flow,mroch/flow,samwgoldman/flow,TiddoLangerak/flow,jamesgpearce/flow,JonathanUsername/flow,nmote/flow,samwgoldman/flow,jamesgpearce/flow,jamesgpearce/flow,popham/flow,popham/flow,ylu1317/flow,facebook/flow,AgentME/flow,AgentME/flow,facebook/flow,mroch/flow,samwgoldman/flow,TiddoLangerak/flow,jamesgpearce/flow,facebook/flow,samwgoldman/flow,AgentME/flow,claudiopro/flow,samwgoldman/flow,TiddoLangerak/flow,claudiopro/flow,nmote/flow,JonathanUsername/flow,mroch/flow,samwgoldman/flow,mroch/flow,gabelevi/flow,facebook/flow,nmote/flow,facebook/flow,JonathanUsername/flow,gabelevi/flow,mroch/flow,popham/flow,facebook/flow,claudiopro/flow,facebook/flow,JonathanUsername/flow,ylu1317/flow,jamesgpearce/flow,gabelevi/flow,gabelevi/flow,samwgoldman/flow,JonathanUsername/flow,popham/flow,ylu1317/flow,jamesgpearce/flow,AgentME/flow,nmote/flow,TiddoLangerak/flow,gabelevi/flow,nmote/flow,popham/flow,gabelevi/flow