Add a section to ABI.rst describing union layout.
Add a section to ABI.rst describing union layout.

Swift SVN r7532

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