Clarify that `UIScheduler` can enqueue work asynchronously
Clarify that `UIScheduler` can enqueue work asynchronously

h/t @neilpa
https://github.com/ReactiveCocoa/ReactiveCocoa/pull/1846#discussion_r281
96057

hilllinux/ReactiveCocoa,liufeigit/ReactiveCocoa,zhiwen1024/ReactiveCocoa,itschaitanya/ReactiveCocoa,j364960953/ReactiveCocoa,loupman/ReactiveCocoa,ztchena/ReactiveCocoa,rpowelll/ReactiveCocoa,BlessNeo/ReactiveCocoa,paulyoung/ReactiveCocoa,LHDsimon/ReactiveCocoa,hilllinux/ReactiveCocoa,jam891/ReactiveCocoa,tiger8888/ReactiveCocoa,dachaoisme/ReactiveCocoa,ioshger0125/ReactiveCocoa,CQXfly/ReactiveCocoa,BrooksWon/ReactiveCocoa,Ray0218/ReactiveCocoa,sugar2010/ReactiveCocoa,brasbug/ReactiveCocoa,Juraldinio/ReactiveCocoa,monkeydbobo/ReactiveCocoa,JackLian/ReactiveCocoa,pzw224/ReactiveCocoa,qq644531343/ReactiveCocoa,GuitarPlayer-Ma/ReactiveCocoa,libiao88/ReactiveCocoa,yoichitgy/ReactiveCocoa,AlanJN/ReactiveCocoa,WEIBP/ReactiveCocoa,hj3938/ReactiveCocoa,huiping192/ReactiveCocoa,zxq3220122/ReactiveCocoa,bensonday/ReactiveCocoa,yizzuide/ReactiveCocoa,andersio/ReactiveCocoa,towik/ReactiveCocoa,kaylio/ReactiveCocoa,terry408911/ReactiveCocoa,xulibao/ReactiveCocoa,DongDongDongDong/ReactiveCocoa,shaohung001/ReactiveCocoa,vincentiss/ReactiveCocoa,walkingsmarts/ReactiveCocoa,WEIBP/ReactiveCocoa,hoanganh6491/ReactiveCocoa,qq644531343/ReactiveCocoa,longv2go/ReactiveCocoa,zzzworm/ReactiveCocoa,almassapargali/ReactiveCocoa,Farteen/ReactiveCocoa,zhigang1992/ReactiveCocoa,msdgwzhy6/ReactiveCocoa,loupman/ReactiveCocoa,lixar/ReactiveCocoa,andersio/ReactiveCocoa,mxxiv/ReactiveCocoa,fanghao085/ReactiveCocoa,jsslai/ReactiveCocoa,icepy/ReactiveCocoa,takeshineshiro/ReactiveCocoa,ceekayel/ReactiveCocoa,richeterre/ReactiveCocoa,icepy/ReactiveCocoa,dachaoisme/ReactiveCocoa,FelixYin66/ReactiveCocoa,yonekawa/ReactiveCocoa,nickcheng/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,esttorhe/ReactiveCocoa,dskatz22/ReactiveCocoa,jrmiddle/ReactiveCocoa,jackywpy/ReactiveCocoa,yaoxiaoyong/ReactiveCocoa,mtxs007/ReactiveCocoa,tonyli508/ReactiveCocoa,mattpetters/ReactiveCocoa,xumaolin/ReactiveCocoa,cnbin/ReactiveCocoa,OneSmallTree/ReactiveCocoa,Ethan89/ReactiveCocoa,windgo/ReactiveCocoa,smilypeda/ReactiveCocoa,beni55/ReactiveCocoa,BlessNeo/ReactiveCocoa,tonyli508/ReactiveCocoa,takeshineshiro/ReactiveCocoa,Pingco/ReactiveCocoa,Ethan89/ReactiveCocoa,eyu1988/ReactiveCocoa,dullgrass/ReactiveCocoa,Farteen/ReactiveCocoa,walkingsmarts/ReactiveCocoa,JackLian/ReactiveCocoa,Ricowere/ReactiveCocoa,libiao88/ReactiveCocoa,PSPDFKit-labs/ReactiveCocoa,Eveian/ReactiveCocoa,msdgwzhy6/ReactiveCocoa,GuitarPlayer-Ma/ReactiveCocoa,yonekawa/ReactiveCocoa,zhenlove/ReactiveCocoa,KuPai32G/ReactiveCocoa,zzqiltw/ReactiveCocoa,on99/ReactiveCocoa,cogddo/ReactiveCocoa,liufeigit/ReactiveCocoa,yangyangluoluo/ReactiveCocoa,chieryw/ReactiveCocoa,shaohung001/ReactiveCocoa,loupman/ReactiveCocoa,xumaolin/ReactiveCocoa,isghe/ReactiveCocoa,CQXfly/ReactiveCocoa,leelili/ReactiveCocoa,200895045/ReactiveCocoa,200895045/ReactiveCocoa,zhukaixy/ReactiveCocoa,shuxiashusheng/ReactiveCocoa,RuiAAPeres/ReactiveCocoa,tzongw/ReactiveCocoa,OneSmallTree/ReactiveCocoa,rpowelll/ReactiveCocoa,valleyman86/ReactiveCocoa,howandhao/ReactiveCocoa,SanChain/ReactiveCocoa,ioshger0125/ReactiveCocoa,JohnJin007/ReactiveCocoa,cogddo/ReactiveCocoa,tonyli508/ReactiveCocoa,stupidfive/ReactiveCocoa,RuiAAPeres/ReactiveCocoa,ShawnLeee/ReactiveCocoa,stevielu/ReactiveCocoa,dz1111/ReactiveCocoa,yytong/ReactiveCocoa,llb1119/test,chieryw/ReactiveCocoa,nickcheng/ReactiveCocoa,nikita-leonov/ReactiveCocoa,Pingco/ReactiveCocoa,sandyway/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,mxxiv/ReactiveCocoa,mtxs007/ReactiveCocoa,alvinvarghese/ReactiveCocoa,LHDsimon/ReactiveCocoa,ddc391565320/ReactiveCocoa,yaoxiaoyong/ReactiveCocoa,sdhzwm/ReactiveCocoa,jeelun/ReactiveCocoa,tzongw/ReactiveCocoa,kevin-zqw/ReactiveCocoa,sandyway/ReactiveCocoa,kiurentu/ReactiveCocoa,bensonday/ReactiveCocoa,Ethan89/ReactiveCocoa,Liquidsoul/ReactiveCocoa,zzqiltw/ReactiveCocoa,natestedman/ReactiveCocoa,FelixYin66/ReactiveCocoa,vincentiss/ReactiveCocoa,jrmiddle/ReactiveCocoa,hilllinux/ReactiveCocoa,imkerberos/ReactiveCocoa,CQXfly/ReactiveCocoa,cogddo/ReactiveCocoa,OnTheWay1988/ReactiveCocoa,BrooksWon/ReactiveCocoa,yytong/ReactiveCocoa,Eveian/ReactiveCocoa,sujeking/ReactiveCocoa,Juraldinio/ReactiveCocoa,cstars135/ReactiveCocoa,wangqi211/ReactiveCocoa,chao95957/ReactiveCocoa,zhenlove/ReactiveCocoa,sdhzwm/ReactiveCocoa,natestedman/ReactiveCocoa,Liquidsoul/ReactiveCocoa,yangyangluoluo/ReactiveCocoa,msdgwzhy6/ReactiveCocoa,bscarano/ReactiveCocoa,yytong/ReactiveCocoa,clg0118/ReactiveCocoa,xiaobing2007/ReactiveCocoa,eyu1988/ReactiveCocoa,cnbin/ReactiveCocoa,ikesyo/ReactiveCocoa,yangshengchaoios/ReactiveCocoa,takeshineshiro/ReactiveCocoa,yangshengchaoios/ReactiveCocoa,paulyoung/ReactiveCocoa,esttorhe/ReactiveCocoa,fanghao085/ReactiveCocoa,kaylio/ReactiveCocoa,paulyoung/ReactiveCocoa,sujeking/ReactiveCocoa,xiaobing2007/ReactiveCocoa,yangshengchaoios/ReactiveCocoa,luerhouhou/ReactiveCocoa,CCOOOOLL/ReactiveCocoa,bscarano/ReactiveCocoa,OneSmallTree/ReactiveCocoa,terry408911/ReactiveCocoa,valleyman86/ReactiveCocoa,fhchina/ReactiveCocoa,zhukaixy/ReactiveCocoa,dskatz22/ReactiveCocoa,zhukaixy/R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