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

apparentlymart/golang,apparentlymart/golang,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,dsriram/golang,dsriram/golang,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,gnanderson/go,dsriram/golang,apparentlymart/golang,gnanderson/go,apparentlymart/golang,tv42/old-go,tv42/old-go,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,dsriram/golang,tv42/old-go,gnanderson/go,dsriram/golang,pombredanne/go-deleteme,gnanderson/go,gnanderson/go,dsriram/golang,apparentlymart/golang,dsriram/golang,gnanderson/go