doc/go1.3.txt: mention windows syscall.NewCallbackCDecl

R=golang-codereviews, r
CC=golang-codereviews
https://golang.org/cl/44160044

doc/go1.3.txt

-pull linker i/o into separate liblink C library (CL 35790044)
-misc/dist renamed misc/makerelease (CL 39920043)
+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)