hrtimers: add missing docbook comments to struct hrtimer

/**

 * struct hrtimer - the basic hrtimer structure

 * @node:	red black tree node for time ordered insertion

 * @expires:	the absolute expiry time in the hrtimers internal

 * @_expires:	the absolute expiry time in the hrtimers internal

 *		representation. The time is related to the clock on

 *		which the timer is based.

 *		which the timer is based. Is setup by adding

 *		slack to the _softexpires value. For non range timers

 *		identical to _softexpires.

 * @_softexpires: the absolute earliest expiry time of the hrtimer.

 *		The time which was given as expiry time when the timer

 *		was armed.

 * @function:	timer expiry callback function

 * @base:	pointer to the timer base (per cpu and per clock)

 * @state:	state information (See bit values above)