doc: add freebsd items

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/71000043

doc/go1.3.txt

@@ -15,11 +15,14 @@ net/http: add optional Server.ConnState callback (CL 69260044)
 net/http: use TCP Keep-Alives on DefaultTransport's connections (CL 68330046)
 net/http: use TCP keep-alives for ListenAndServe and ListenAndServeTLS (CL 48300043)
 net: add Dialer.KeepAlive option (CL 68380043)
+net: enable fast socket creation using SOCK_CLOEXEC and Accept4 on FreeBSD 10 (69100043)
 os/exec: fix Command with relative paths (CL 59580044)
 runtime/debug: add SetPanicOnFault (CL 66590044)
 runtime: output how long goroutines are blocked (CL 50420043)
 sync: add Pool (CL 41860043, 46010043)
+syscall: add Accept4 for freebsd (CL 68880043)
 syscall: add NewCallbackCDecl to use for windows callbacks (CL 36180044)
+syscall: add support for FreeBSD 10 (CL 56770044, 56980043)
 testing: add b.RunParallel function (CL 57270043)
 testing: diagnose buggy tests that panic(nil) (CL 55780043)
 unicode: upgrade from Unicode 6.2.0 to 6.3.0 (CL 65400044)