drivers: stepper: Cleanup doxygen comments

Cleans up the doxygen comments in the stepper api header. Includes:
- documentation for MICRO_STEP_RES_INDEX
- constistent use of microsteps instead of micro_steps or micro steps
- fix for previously renamed set_target_position refrence

Signed-off-by: Fabian Blatz <fabianblatz@gmail.com>