cmd/go: document -v flag for get

Fixes #16719.

Change-Id: I20550628814e3454f17d6f8ae8b66cce17f09859
Reviewed-on: https://go-review.googlesource.com/27118Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/alldocs.go

@@ -496,6 +496,8 @@
 // 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.
 //
+// The -v flag enables verbose progress and debug output.
+//
 // Get also accepts build flags to control the installation. See 'go help build'.
 //
 // When checking out a new package, get creates the target directory

src/cmd/go/get.go

@@ -43,6 +43,8 @@ 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.
 
+The -v flag enables verbose progress and debug output.
+
 Get also accepts build flags to control the installation. See 'go help build'.
 
 When checking out a new package, get creates the target directory