doc/go1.4: add some cmd/go changes

CC=golang-codereviews
https://golang.org/cl/143650043

Author: rsc

doc/go1.4.txt

@@ -14,6 +14,8 @@ cmd/go: import comments (CL 124940043)
 cmd/go: implement "internal" (CL 120600043)
 cmd/go: implement "generate" (CL 125580044)
 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)
 
 asm: make textflag.h available outside of cmd/ld (CL 128050043)
 crypto/tls: add support for ALPN (RFC 7301) (CL 108710046)