sort: document two undocumented functions
sort: document two undocumented functions

They looked out of place in godoc.
Includes documenting sort stability.

Fixes issue 3356

R=golang-dev, gri, trolleriprofessorn
CC=golang-dev
http://codereview.appspot.com/5855044

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang