command: use NUL for magic usage string

This change also improves documentation.

Signed-off-by: Ophestra <cat@gensokyo.uk>

command/node.go

@@ -6,6 +6,7 @@ import (
 	"strings"
 )
 
+// A node represents a command.
 type node struct {
 	child, next *node
 	name, usage string
@@ -13,13 +14,16 @@ type node struct {
 	out  io.Writer
 	logf LogFunc
 
+	// Names of commands preceding node.
 	prefix []string
+	// Short user-facing representations of flags received by node.
 	suffix strings.Builder
 
 	f   HandlerFunc
 	set *flag.FlagSet
 }
 
+// adopt adds v as the last child of n.
 func (n *node) adopt(v *node) bool {
 	if n.child != nil {
 		return n.child.append(v)
@@ -28,6 +32,7 @@ func (n *node) adopt(v *node) bool {
 	return true
 }
 
+// append adds v as the last sibling of n.
 func (n *node) append(v *node) bool {
 	if n.name == v.name {
 		return false