[hardware_interface::RobotHW] doc: update read and write, fix: group names

[fjp]

This PR is split from the large PR #433 and contains the following changes:

• Update brief and detailes of read and write methods
• Fix doxygen group names and add new group name "Control Loop" to read and write methods. Removes these methods from group Hardware Interface Switching
• Remove too many details of class description in PR #433 -> new PR
• Remove too many details of init method in PR #433 -> new PR
• Apply suggestions from code review in #433

It is related to ros-controls/ros_controllers/#474.

Co-Authored-By: Matt Reynolds mtreynolds@uwaterloo.ca

[fjp]

Thanks @matthew-reynolds reviewing this. I've applied your suggestions, please let me know if something else needs to be fixed. After that I'll continue with the rest mentioned in #433.