internal/scan: update govulncheck docs

Update docs for the `govulncheck -show` and `govulncheck -scan` flags.

Change-Id: I4961c8ba993c21bbca942fee26a6e341e87dec8c
Reviewed-on: https://go-review.googlesource.com/c/vuln/+/508918
Auto-Submit: Julie Qiu <julieqiu@google.com>
Reviewed-by: Ian Cottrell <iancottrell@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Julie Qiu <julieqiu@google.com>
Run-TryBot: Julie Qiu <julieqiu@google.com>

Author: julieqiu

cmd/govulncheck/testdata/usage.ct

@@ -16,10 +16,11 @@ Usage:
     	output JSON
   -mode string
     	supports source or binary (default "source")
-  -scan-level string
+  -scan string
     	set the scanning level desired, one of module, package or symbol (default "symbol")
   -show list
-    	enable display of additional information specified by list
+    	enable display of additional information specified by the comma separated list
+    	The only supported value is 'traces'
   -tags list
     	comma-separated list of build tags
   -test
@@ -45,10 +46,11 @@ Usage:
     	output JSON
   -mode string
     	supports source or binary (default "source")
-  -scan-level string
+  -scan string
     	set the scanning level desired, one of module, package or symbol (default "symbol")
   -show list
-    	enable display of additional information specified by list
+    	enable display of additional information specified by the comma separated list
+    	The only supported value is 'traces'
   -tags list
     	comma-separated list of build tags
   -test

internal/scan/flags.go

@@ -47,8 +47,8 @@ func parseFlags(cfg *config, stderr io.Writer, args []string) error {
 	flags.StringVar(&cfg.db, "db", "https://vuln.go.dev", "vulnerability database `url`")
 	flags.StringVar(&cfg.mode, "mode", modeSource, "supports source or binary")
 	flags.Var(&tagsFlag, "tags", "comma-separated `list` of build tags")
-	flags.Var(&showFlag, "show", "enable display of additional information specified by `list`")
-	scanLevel := flags.String("scan-level", "symbol", "set the scanning level desired, one of module, package or symbol")
+	flags.Var(&showFlag, "show", "enable display of additional information specified by the comma separated `list`\nThe only supported value is 'traces'")
+	scanLevel := flags.String("scan", "symbol", "set the scanning level desired, one of module, package or symbol")
 	flags.Usage = func() {
 		fmt.Fprint(flags.Output(), `Govulncheck reports known vulnerabilities in dependencies.