Take care for documentation

[destogl]

@bmagyar This assumes that command_interfaces_ comes ordered as you requested it, but I did not find any documentation explicitly mentioning this, that's why I went with the get_ordered_interfaces thing.

we could just explicitely mention that the itnerfaces are provided in the order they were requested, that would simplify things.

Originally posted by @v-lopez in https://github.com/ros-controls/ros2_controllers/pull/99#discussion_r532390821