Since `Thor::HELP_MAPPINGS` contains `-D`, so `bin/rails -D` show
rails's help. But, in Rails 5.0.1, `bin/rails -D` show the description
of rake task. I think that it is better to have the same behavior.
Thor would inadvertantly duplicate the command usage because
of the help method in a command class.
Fixes#26664.
[ Yuji Yaginuma & Kasper Timm Hansen ]