doc/go1.4.txt: flags can now be set at most once

CC=golang-codereviews
https://golang.org/cl/161820043

doc/go1.4.txt

@@ -23,6 +23,7 @@ asm: make textflag.h available outside of cmd/ld (CL 128050043)
 bufio: handling of empty tokens at EOF changed, may require scanner change (CL 145390043)
 crypto/tls: add support for ALPN (RFC 7301) (CL 108710046)
 crypto/tls: support programmatic selection of server certificates (CL 107400043)
+flag: it is now an error to set a flag multiple times (CL 156390043)
 fmt: print type *map[T]T as &map[k:v] (CL 154870043)
 encoding/gob: remove unsafe (CL 102680045)
 misc: deleted editor support; refer to https://code.google.com/p/go-wiki/wiki/IDEsAndTextEditorPlugins instead (CL 105470043)