cmd/go: clarify go get documentation

Make the documentation for `go get` match the documentation for `go
install`, since `go get` essentially invokes `go install`.

Update #15825.

Change-Id: I374d80efd301814b6d98b86b7a4a68dd09704c92
Reviewed-on: https://go-review.googlesource.com/23925
Reviewed-by: Andrew Gerrand <adg@golang.org>

src/cmd/go/alldocs.go

@@ -473,8 +473,8 @@ Usage:
 
 	go get [-d] [-f] [-fix] [-insecure] [-t] [-u] [build flags] [packages]
 
-Get downloads and installs the packages named by the import paths,
-along with their dependencies.
+Get downloads the packages named by the import paths, along with their
+dependencies. It then installs the named packages, like 'go install'.
 
 The -d flag instructs get to stop after downloading the packages; that is,
 it instructs get not to install the packages.

src/cmd/go/get.go

@@ -19,8 +19,8 @@ var cmdGet = &Command{
 	UsageLine: "get [-d] [-f] [-fix] [-insecure] [-t] [-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.
+Get downloads the packages named by the import paths, along with their
+dependencies. It then installs the named packages, like 'go install'.
 
 The -d flag instructs get to stop after downloading the packages; that is,
 it instructs get not to install the packages.