Document how out-of-range inputs are handled
Document how out-of-range inputs are handled

This changed recently so that overflow and underflow consistently return signed
infinity or zero instead of nil.  (Previously, overflow returned nil, underflow
returned zero.)

Among other benefits:
 * The new behavior distinguishes malformed input (nil) from valid but out-of-range input.
 * The new behavior preserves the sign of the input
 * The new behavior is consistent with how floating-point parsing behaves in other langauges

Resolves rdar://76728925

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