libc/unistd : Add a comment about usleep operates in tick
Since usleep operates in tick, It always works by counting in tick.
* due to the scheduling of other activity by the system.
*
* The 'usec' argument must be less than 1,000,000. If the value of
- * 'usec' is 0, then the call has no effect.
+ * 'usec' is 0, then the call has no effect. If CONFIG_SCHED_TICKLESS
+ * is disabled and the value of 'usec' is less than the value of tick,
+ * CONFIG_USEC_PER_TICK, then this function operates with the value of tick.
*
* If a SIGALRM signal is generated for the calling process during
* execution of usleep() and if the SIGALRM signal is being ignored or
