Document the additional --names argument that mix help can take

Author: pwoolcoc

lib/mix/lib/mix/tasks/help.ex

@@ -8,8 +8,9 @@ defmodule Mix.Tasks.Help do
 
   ## Arguments
 
-      mix help      - prints all tasks and their shortdoc
-      mix help TASK - prints full docs for the given task
+      mix help         - prints all tasks and their shortdoc
+      mix help --names - prints all task names
+      mix help TASK    - prints full docs for the given task
 
   ## Colors