doc: add cmd/gofmt entry to go1.3.txt

R=golang-codereviews, minux.ma
CC=golang-codereviews
https://golang.org/cl/52240043

doc/go1.3.txt

+cmd/gofmt: remove -tabwidth and -tabs flags (CL 52170043)
 liblink: pull linker i/o into separate liblink C library (CL 35790044)
 misc/dist: renamed misc/makerelease (CL 39920043)
 syscall: add NewCallbackCDecl to use for windows callbacks (CL 36180044)