cmd/go: update 'go get' help message

It accepts all the build flags.
Say that instead of making a copy that will go stale.

Fixes #4742.

R=golang-dev, minux.ma
CC=golang-dev
https://golang.org/cl/7229081

src/cmd/go/doc.go

@@ -224,14 +224,11 @@ Download and install packages and dependencies
 
 Usage:
 
-	go get [-a] [-d] [-fix] [-n] [-p n] [-u] [-v] [-x] [packages]
+	go get [-d] [-fix] [-u] [build flags] [packages]
 
 Get downloads and installs the packages named by the import paths,
 along with their dependencies.
 
-The -a, -n, -v, -x, and -p flags have the same meaning as in 'go build'
-and 'go install'.  See 'go help build'.
-
 The -d flag instructs get to stop after downloading the packages; that is,
 it instructs get not to install the packages.
 
@@ -242,6 +239,9 @@ The -u flag instructs get to use the network to update the named packages
 and their dependencies.  By default, get uses the network to check out
 missing packages but does not use it to look for updates to existing packages.
 
+Get also accepts all the flags in the 'go build' and 'go install' commands,
+to control the installation. See 'go help build'.
+
 When checking out or updating a package, get looks for a branch or tag
 that matches the locally installed version of Go. The most important
 rule is that if the local installation is running version "go1", get

src/cmd/go/get.go

@@ -18,15 +18,12 @@ import (
 )
 
 var cmdGet = &Command{
-	UsageLine: "get [-a] [-d] [-fix] [-n] [-p n] [-u] [-v] [-x] [packages]",
+	UsageLine: "get [-d] [-fix] [-u] [build flags] [packages]",
 	Short:     "download and install packages and dependencies",
 	Long: `
 Get downloads and installs the packages named by the import paths,
 along with their dependencies.
 
-The -a, -n, -v, -x, and -p flags have the same meaning as in 'go build'
-and 'go install'.  See 'go help build'.
-
 The -d flag instructs get to stop after downloading the packages; that is,
 it instructs get not to install the packages.
 
@@ -37,6 +34,9 @@ The -u flag instructs get to use the network to update the named packages
 and their dependencies.  By default, get uses the network to check out
 missing packages but does not use it to look for updates to existing packages.
 
+Get also accepts all the flags in the 'go build' and 'go install' commands,
+to control the installation. See 'go help build'.
+
 When checking out or updating a package, get looks for a branch or tag
 that matches the locally installed version of Go. The most important
 rule is that if the local installation is running version "go1", get