cmd/doc: adding validation before adding comment marker

Previous fix in issue #20929 for adding comment marker does
not check whether string field have // prefix or not.
This commit ensures string field does not contain // before adding
prefix to the line. Test also included in this commit.

Fixes #40992

Change-Id: Ibc5e8ef147eeb2ed732fb9e19815c8b21fcfb2ab
Reviewed-on: https://go-review.googlesource.com/c/go/+/251237


Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Robert Griesemer <gri@golang.org>
Trust: Dmitri Shuralyov <dmitshur@golang.org>