Fix threshold for PInvoke path in Buffer.Memmove
Fix threshold for PInvoke path in Buffer.Memmove

Threshold for PInvoke path in Buffer.Memmove was way too low. I suspect it was tuned based on the debug builds. I have updated it to be
at the lower end of the break-even point range, based on measurements on several different types of hardware. The best improvements are
on i7-3520M: 64-byte aligned memory blocks - 35% faster, 128 byte aligned memory blocks - 17% faster.

Also:
- Delete misleading comment about PInvoke overhead
- Add comment related to #430

gregkalapos/corert,botaberg/corert,schellap/corert,manu-silicon/corert,sandreenko/corert,kyulee1/corert,manu-silicon/corert,manu-silicon/corert,tijoytom/corert,mjp41/corert,sandreenko/corert,sandreenko/corert,gregkalapos/corert,yizhang82/corert,gregkalapos/corert,manu-silicon/corert,botaberg/corert,tijoytom/corert,mjp41/corert,krytarowski/corert,sandreenko/corert,yizhang82/corert,botaberg/corert,schellap/corert,mjp41/corert,shrah/corert,tijoytom/corert,shrah/corert,manu-silicon/corert,yizhang82/corert,krytarowski/corert,schellap/corert,tijoytom/corert,yizhang82/corert,mjp41/corert,mjp41/corert,schellap/corert,krytarowski/corert,gregkalapos/corert,schellap/corert,kyulee1/corert,botaberg/corert,kyulee1/corert,shrah/corert,kyulee1/corert,shrah/corert,krytarowski/corert