Document the "pure" attribute.
Document the "pure" attribute.

Swift SVN r19657

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imonaka/swift,djwbrown/swift,jmgc/swift,roambotics/swift,swiftix/swift,devincoughlin/swift,JaSpa/swift,tinysun212/swift-windows,gmilos/swift,benlangmuir/swift,harlanhaskins/swift,OscarSwanros/swift,dreamsxin/swift,aschwaighofer/swift,aschwaighofer/swift,brentdax/swift,cbrentharris/swift,swiftix/swift.old,airspeedswift/swift,ben-ng/swift,shajrawi/swift,zisko/swift,tkremenek/swift,SwiftAndroid/swift,shajrawi/swift,sdulal/swift,glessard/swift,austinzheng/swift,gribozavr/swift,sdulal/swift,danielmartin/swift,sschiau/swift,kstaring/swift,ahoppen/swift,amraboelela/swift,xedin/swift,hughbe/swift,djwbrown/swift,KrishMunot/swift,benlangmuir/swift,khizkhiz/swift,amraboelela/swift,Jnosh/swift,modocache/swift,airspeedswift/swift,djwbrown/swift,hughbe/swift,ken0nek/swift,airspeedswift/swift,therealbnut/swift,cbrentharris/swift,atrick/swift,SwiftAndroid/swift,stephentyrone/swift,brentdax/swift,frootloops/swift,swiftix/swift.old,sschiau/swift,Ivacker/swift,frootloops/swift,djwbrown/swift,return/swift,tjw/swift,modocache/swift,KrishMunot/swift,IngmarStein/swift,glessard/swift,tjw/swift,djwbrown/swift,jckarter/swift,bitjammer/swift,khizkhiz/swift,zisko/swift,kstaring/swift,parkera/swift,gmilos/swift,amraboelela/swift,jtbandes/swift,cbrentharris/swift,OscarSwanros/swift,calebd/swift,SwiftAndroid/swift,Jnosh/swift,mightydeveloper/swift,deyton/swift,xedin/swift,swiftix/swift.old,gribozavr/swift,alblue/swift,therealbnut/swift,kstaring/swift,ben-ng/swift,sdulal/swift,Jnosh/swift,jckarter/swift,xwu/swift,tjw/swift,JGiola/swift,jmgc/swift,sdulal/swift,CodaFi/swift,swiftix/swift.old,tkremenek/swift,hughbe/swift,MukeshKumarS/Swift,jmgc/swift,felix91gr/swift,swiftix/swift,johnno1962d/swift,gregomni/swift,russbishop/swift,JaSpa/swift,mightydeveloper/swift,airspeedswift/swift,LeoShimonaka/swift,calebd/swift,jopamer/swift,zisko/swift,brentdax/swift,tjw/swift,roambotics/swift,aschwaighofer/swift,swiftix/swift.old,apple/swift,swiftix/swift,tkremenek/swift,uasys/swift,devincoughlin/swift,swiftix/swift,amraboelela/swift,stephentyrone/swift,jmgc/swift,khizkhiz/swift,kusl/swift,shahmishal/swift,tinysun212/swift-windows,swiftix/swift,kstaring/swift,adrfer/swift,lorentey/swift,kstaring/swift,apple/swift,natecook1000/swift,cbrentharris/swift,huonw/swift,ken0nek/swift,xedin/swift,cbrentharris/swift,danielmartin/swift,cbrentharris/swift,KrishMunot/swift,natecook1000/swift,kperryua/swift,emilstahl/swift,manavgabhawala/swift,xwu/swift,tjw/swift,milseman/swift,practicalswift/swift,bitjammer/swift,JaSpa/swift,aschwaighofer/swift,codestergit/swift,johnno1962d/swift,practicalswift/swift,karwa/swift,stephentyrone/swift,xwu/swift,atrick/swift,zisko/swift,ahoppen/swift,huonw/swift,SwiftAndroid/swift,xwu/swift,xedin/swift,IngmarStein/swift,arvedviehweger/swift,rudkx/swift,benlangmuir/swift,frootloops/swift,kstaring/swift,sdulal/swift,lorentey/swift,alblue/swift,kentya6/swift,parkera/swift,jckarter/swift,jtbandes/swift,alblue/swift,swiftix/swift.old,manavgabhawala/swift,shajrawi/swift,kusl/swift,rudkx/swift,jtbandes/swift,jmgc/swift,airspeedswift/swift,djwbrown/swift,lorentey/swift,atrick/swift,emilstahl/swift,JaSpa/swift,tinysun212/swift-windows,JGiola/swift,kperryua/swift,cbrentharris/swift,IngmarStein/swift,KrishMunot/swift,kperryua/swift,roambotics/swift,ken0nek/swift,shahmishal/swift,return/swift,roambotics/swift,ben-ng/swift,shahmishal/swift,IngmarStein/swift,zisko/swift,zisko/swift,mightydeveloper/swift,xwu/swift,LeoShimonaka/swift,jtbandes/swift,shahmishal/swift,jtbandes/swift,allevato/swift,manavgabhawala/swift,gottesmm/swift,jckarter/swift,austinzheng/swift,Ivacker/swift,cbrentharris/swift,kperryua/swift,therealbnut/swift,jtbandes/swift,danielmartin/swift,hooman/swift,sdulal/swift,SwiftAndroid/swift,kusl/swift,LeoShimonaka/swift,Jnosh/swift,djwbrown/swift,kusl/swift,jmgc/swift,therealbnut/swift,russbishop/swift,lorentey/swift,IngmarStein/swift,kentya6/swift,ahoppen/swift,mightydeveloper/swift,felix91gr/swift,karwa/swift,stephentyrone/swift,tkremenek/swift,milseman/swift,jopamer/swift,milseman/swift,harlanhaskins/swift,OscarSwanros/swift,xedin/swift,arvedviehweger/swift,allevato/swift,gregomni/swift,alblue/swift,tkremenek/swift,therealbnut/swift,natecook1000/swift,johnno1962d/swift,allevato/swift,jckarter/swift,Ivacker/swift,swiftix/swift,codestergit/swift,arvedviehweger/swift,slavapestov/swift,ahoppen/swift,frootloops/swift,calebd/swift,adrfer/swift,rudkx/swift,danielmartin/swift,modocache/swift,parkera/swift,gregomni/swift,CodaFi/swift,natecook1000/swift,allevato/swift,nathawes/swift,Ivacker/swift,deyton/swift,dreamsxin/swift,tardieu/swift,roambotics/swift,xwu/swift,practicalswift/swift,tjw/swift,lorentey/swift,codestergit/swift,gmilos/swift,natecook1000/swift,gribozavr/swift,tardieu/swift,khizkhiz/swift,xedin/swift,felix91gr/swift