cmd/go: clarify the -o testflag is only for copying the binary

Fixes #74769

Change-Id: Iebbaea8fb1a25e30a541a827815def9e269a8135
Reviewed-on: https://go-review.googlesource.com/c/go/+/709255
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Ian Alexander <jitsu@google.com>
Reviewed-by: Michael Matloob <matloob@google.com>

Author: seankhliao

src/cmd/go/alldocs.go

@@ -162,7 +162,7 @@ In addition to the build flags, the flags handled by 'go test' itself are:
 	    Also emits build output in JSON. See 'go help buildjson'.
 
 	-o file
-	    Compile the test binary to the named file.
+	    Save a copy of the test binary to the named file.
 	    The test still runs (unless -c or -i is specified).
 	    If file ends in a slash or names an existing directory,
 	    the test is written to pkg.test in that directory.