Add documentation for exact object types (fixes #2524)
Add documentation for exact object types (fixes #2524)

Summary:
Issue #2524

Add documentation for `{|...|}` and `$Exact<...>`.
Closes https://github.com/facebook/flow/pull/2525

Reviewed By: samwgoldman

Differential Revision: D3926156

Pulled By: jeffmo

fbshipit-source-id: efc34e1ea23f33c06b6ffab31433802dec702599

JonathanUsername/flow,nmote/flow,ForbesLindesay/flow,ylu1317/flow,JonathanUsername/flow,popham/flow,AgentME/flow,popham/flow,TiddoLangerak/flow,fletcherw/flow,jgrund/flow,nmote/flow,jgrund/flow,gabelevi/flow,nmote/flow,fletcherw/flow,TiddoLangerak/flow,popham/flow,gabelevi/flow,gabelevi/flow,jamesgpearce/flow,facebook/flow,jamesgpearce/flow,ylu1317/flow,samwgoldman/flow,ForbesLindesay/flow,samwgoldman/flow,MichaelDeBoey/flow,MichaelDeBoey/flow,nmote/flow,jamesgpearce/flow,AprilArcus/flow,ylu1317/flow,MichaelDeBoey/flow,ForbesLindesay/flow,gabelevi/flow,jamesgpearce/flow,samwgoldman/flow,jgrund/flow,popham/flow,samwgoldman/flow,claudiopro/flow,mroch/flow,claudiopro/flow,facebook/flow,ylu1317/flow,claudiopro/flow,ForbesLindesay/flow,popham/flow,facebook/flow,ForbesLindesay/flow,jgrund/flow,mroch/flow,claudiopro/flow,JonathanUsername/flow,jamesgpearce/flow,JonathanUsername/flow,facebook/flow,AprilArcus/flow,fletcherw/flow,TiddoLangerak/flow,AgentME/flow,popham/flow,mroch/flow,JonathanUsername/flow,AprilArcus/flow,fletcherw/flow,gabelevi/flow,mroch/flow,nmote/flow,TiddoLangerak/flow,mroch/flow,jamesgpearce/flow,gabro/flow,AgentME/flow,AgentME/flow,MichaelDeBoey/flow,MichaelDeBoey/flow,ylu1317/flow,gabelevi/flow,fletcherw/flow,TiddoLangerak/flow,samwgoldman/flow,jgrund/flow,jamesgpearce/flow,jgrund/flow,ForbesLindesay/flow,gabelevi/flow,facebook/flow,facebook/flow,gabro/flow,AprilArcus/flow,gabro/flow,claudiopro/flow,fletcherw/flow,AprilArcus/flow,JonathanUsername/flow,facebook/flow,nmote/flow,popham/flow,mroch/flow,gabro/flow,nmote/flow,samwgoldman/flow,samwgoldman/flow,gabro/flow,gabro/flow,JonathanUsername/flow,mroch/flow,AgentME/flow,AprilArcus/flow