sched/dl/Documentation: Clarify indexing notation

 arrival time r_j (the time when the job starts), an amount of computation

 time c_j needed to finish the job, and a job absolute deadline d_j, which

 is the time within which the job should be finished. The maximum execution

 time max_j{c_j} is called "Worst Case Execution Time" (WCET) for the task.

 time max{c_j} is called "Worst Case Execution Time" (WCET) for the task.

 A real-time task can be periodic with period P if r_{j+1} = r_j + P, or

 sporadic with minimum inter-arrival time P is r_{j+1} >= r_j + P. Finally,

 d_j = r_j + D, where D is the task's relative deadline.

 WCET and its period (or minimum inter-arrival time), and represents

 the fraction of CPU time needed to execute the task.

 If the total utilization sum_i(WCET_i/P_i) is larger than M (with M equal

 If the total utilization U=sum(WCET_i/P_i) is larger than M (with M equal

 to the number of CPUs), then the scheduler is unable to respect all the

 deadlines.

 Note that total utilization is defined as the sum of the utilizations

 More precisely, it can be proven that using a global EDF scheduler the

 maximum tardiness of each task is smaller or equal than

	((M − 1) · WCET_max − WCET_min)/(M − (M − 2) · U_max) + WCET_max

 where WCET_max = max_i{WCET_i} is the maximum WCET, WCET_min=min_i{WCET_i}

 is the minimum WCET, and U_max = max_i{WCET_i/P_i} is the maximum utilization.

 where WCET_max = max{WCET_i} is the maximum WCET, WCET_min=min{WCET_i}

 is the minimum WCET, and U_max = max{WCET_i/P_i} is the maximum utilization.

 If M=1 (uniprocessor system), or in case of partitioned scheduling (each

 real-time task is statically assigned to one and only one CPU), it is

 of the tasks running on such a CPU is smaller or equal than 1.

 If D_i != P_i for some task, then it is possible to define the density of

 a task as WCET_i/min{D_i,P_i}, and EDF is able to respect all the deadlines

 of all the tasks running on a CPU if the sum sum_i WCET_i/min{D_i,P_i} of the

 of all the tasks running on a CPU if the sum sum(WCET_i/min{D_i,P_i}) of the

 densities of the tasks running on such a CPU is smaller or equal than 1

 (notice that this condition is only sufficient, and not necessary).