cmd/govulncheck: update docs
cmd/govulncheck: update docs

Change-Id: Ifdeaca46b2242ea7ffa33382f2d496e9ee92b2a9
Reviewed-on: https://go-review.googlesource.com/c/vuln/+/427586
Run-TryBot: Julie Qiu <e6cb64861edcc38971590ff27c111e0e672de045@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Tatiana Bradley <ed5904c3174de8861076818d9fdd7f7c949a16e6@golang.org>
Reviewed-by: Julie Qiu <e6cb64861edcc38971590ff27c111e0e672de045@google.com>
Auto-Submit: Julie Qiu <e6cb64861edcc38971590ff27c111e0e672de045@google.com>
Run-TryBot: Tatiana Bradley <ed5904c3174de8861076818d9fdd7f7c949a16e6@golang.org>

golang/vuln,golang/vuln