cmd/go: clarify stderr behavior for 'go help mod download'

Fixes #36297

Change-Id: I2623e9ee386644eddfa9a348ba368c5f1a81f893
Reviewed-on: https://go-review.googlesource.com/c/go/+/213134
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>

src/cmd/go/alldocs.go

@@ -1023,7 +1023,9 @@
 // execution. The "go mod download" command is useful mainly for pre-filling
 // the local cache or to compute the answers for a Go module proxy.
 //
-// By default, download reports errors to standard error but is otherwise silent.
+// By default, download writes nothing to standard output. It may print progress
+// messages and errors to standard error.
+//
 // The -json flag causes download to print a sequence of JSON objects
 // to standard output, describing each downloaded module (or failure),
 // corresponding to this Go struct:

src/cmd/go/internal/modcmd/download.go

@@ -30,7 +30,9 @@ The go command will automatically download modules as needed during ordinary
 execution. The "go mod download" command is useful mainly for pre-filling
 the local cache or to compute the answers for a Go module proxy.
 
-By default, download reports errors to standard error but is otherwise silent.
+By default, download writes nothing to standard output. It may print progress
+messages and errors to standard error.
+
 The -json flag causes download to print a sequence of JSON objects
 to standard output, describing each downloaded module (or failure),
 corresponding to this Go struct: