doc/go1.2.txt: stable sort

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/10761048

doc/go1.2.txt

@@ -22,4 +22,5 @@ fmt: indexed access to arguments in Printf etc. (CL 9680043).
 go/build: support including C++ code with cgo (CL 8248043).
 io: Copy prioritizes WriterTo over ReaderFrom (CL 9462044).
 net: new build tag netgo for building a pure Go net package (CL 7100050).
+sort: new Stable function provides stable sort (CL 9612044).
 testing: AllocsPerRun is now quantized to an integer (the type is still float64) (CL 9837049).