Document floating point descriptions.
Document floating point descriptions.

Fixes <rdar://problem/51409021>.

tkremenek/swift,ahoppen/swift,rudkx/swift,glessard/swift,benlangmuir/swift,atrick/swift,gregomni/swift,xwu/swift,atrick/swift,xwu/swift,parkera/swift,parkera/swift,roambotics/swift,JGiola/swift,glessard/swift,rudkx/swift,hooman/swift,hooman/swift,roambotics/swift,glessard/swift,parkera/swift,gregomni/swift,ahoppen/swift,ahoppen/swift,parkera/swift,glessard/swift,tkremenek/swift,parkera/swift,rudkx/swift,apple/swift,rudkx/swift,tkremenek/swift,glessard/swift,glessard/swift,ahoppen/swift,ahoppen/swift,gregomni/swift,gregomni/swift,benlangmuir/swift,hooman/swift,JGiola/swift,benlangmuir/swift,rudkx/swift,atrick/swift,JGiola/swift,xwu/swift,tkremenek/swift,parkera/swift,xwu/swift,roambotics/swift,roambotics/swift,tkremenek/swift,apple/swift,xwu/swift,apple/swift,apple/swift,apple/swift,ahoppen/swift,parkera/swift,benlangmuir/swift,JGiola/swift,gregomni/swift,benlangmuir/swift,roambotics/swift,parkera/swift,tkremenek/swift,apple/swift,gregomni/swift,JGiola/swift,rudkx/swift,tkremenek/swift,JGiola/swift,hooman/swift,hooman/swift,roambotics/swift,hooman/swift,hooman/swift,atrick/swift,atrick/swift,xwu/swift,benlangmuir/swift,xwu/swift,atrick/swift