Merge branch 'certmagic-refactor' into v2

cmd/commands.go

@@ -274,6 +274,8 @@ directly instead of printing it.`,
 // This function panics if the name is already registered,
 // if the name does not meet the described format, or if
 // any of the fields are missing from cmd.
+//
+// This function should be used in init().
 func RegisterCommand(cmd Command) {
 	if cmd.Name == "" {
 		panic("command name is required")