cmd/govulncheck: clarify exitcode rationale
cmd/govulncheck: clarify exitcode rationale

Change-Id: I771a65120dd35d8bb4c242376f98eea24f7a6763
Reviewed-on: https://go-review.googlesource.com/c/vuln/+/432235
Reviewed-by: Hyang-Ah Hana Kim <d7e82c3c07f92613bcbf9a1e88f6b96c13caa3b0@gmail.com>
Run-TryBot: Zvonimir Pavlinovic <490af24f3def1258d2f7e9b10b20e341f6c68320@google.com>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/vuln,golang/vuln