command: improve help command docs

When generating implicit usage, mention help if there is a help command
defined. Document the hidden -a/--all flag to the help command.

help.go

@@ -23,8 +23,10 @@ import (
 func HelpCommand(topics []HelpTopic) *C {
 	cmd := &C{
 		Name:  "help",
-		Usage: "[topic/command]",
-		Help:  `Print help for the specified command or topic.`,
+		Usage: "[-a|--all] [topic/command]",
+		Help: `Print help for the specified command or topic.
+
+With -a or --all, also show help for unlisted commands and private flags.`,
 
 		CustomFlags: true,
 

utils.go

@@ -42,7 +42,10 @@ func (c *C) usageLines(flags HelpFlags) []string {
 			tag = joinSpace(tag, "<command>")
 		}
 		if tag != "" {
-			return []string{tag}
+			lines = append(lines, tag)
+		}
+		if hc := c.FindSubcommand("help"); hc != nil && hc.Runnable() {
+			lines = append(lines, "help")
 		}
 	}
 	return lines