doc/go1.4.txt: GOOS suffixes require underscores to act as build tags

CC=golang-codereviews
https://golang.org/cl/151490043

doc/go1.4.txt

@@ -17,6 +17,7 @@ cmd/go: disallow C sources except when using cgo (CL 149720043)
 cmd/go: add test -o flag (CL 149070043)
 cmd/go: redefine build -a to skip standard library in releases (CL 151730045)
 cmd/go: compile and link all _test.go files during 'go test', even in packages where there are no Test functions (CL 150980043)
+cmd/go: (via go/build): a GOOS prefix acts as a tag only if preceded by an underscore. this is a breaking change. (CL 147690043)
 
 asm: make textflag.h available outside of cmd/ld (CL 128050043)
 crypto/tls: add support for ALPN (RFC 7301) (CL 108710046)