chore: Add documentation for dryrun parameter (#2311)

The `dryrun` paramater's documentation is unhelpful (`dryrun mode`).

In the absense of proper documentation, [this definition from a comment on this topic](nektos/act#268 (comment)) should be used.

Fixes: nektos/act#268

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

cmd/root.go

@@ -78,7 +78,7 @@ func Execute(ctx context.Context, version string) {
 	rootCmd.PersistentFlags().BoolVar(&input.jsonLogger, "json", false, "Output logs in json format")
 	rootCmd.PersistentFlags().BoolVar(&input.logPrefixJobID, "log-prefix-job-id", false, "Output the job id within non-json logs instead of the entire name")
 	rootCmd.PersistentFlags().BoolVarP(&input.noOutput, "quiet", "q", false, "disable logging of output from steps")
-	rootCmd.PersistentFlags().BoolVarP(&input.dryrun, "dryrun", "n", false, "dryrun mode")
+	rootCmd.PersistentFlags().BoolVarP(&input.dryrun, "dryrun", "n", false, "disable container creation, validates only workflow correctness")
 	rootCmd.PersistentFlags().StringVarP(&input.secretfile, "secret-file", "", ".secrets", "file with list of secrets to read from (e.g. --secret-file .secrets)")
 	rootCmd.PersistentFlags().StringVarP(&input.varfile, "var-file", "", ".vars", "file with list of vars to read from (e.g. --var-file .vars)")
 	rootCmd.PersistentFlags().BoolVarP(&input.insecureSecrets, "insecure-secrets", "", false, "NOT RECOMMENDED! Doesn't hide secrets while printing logs.")