sched/deadline: Update GRUB description in the documentation

Update the details of GRUB to reflect the updated logic.

Signed-off-by: Vineeth Pillai (Google) <[e-mail]>
Signed-off-by: Peter Zijlstra (Intel) <[e-mail]>
Reviewed-by: Daniel Bristot de Oliveira <[e-mail]>
Acked-by: Juri Lelli <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/scheduler/sched-deadline.rst

···
   - Total bandwidth (this_bw): this is the sum of all tasks "belonging" to the
     runqueue, including the tasks in Inactive state.
 
+  - Maximum usable bandwidth (max_bw): This is the maximum bandwidth usable by
+    deadline tasks and is currently set to the RT capacity.
+
 
  The algorithm reclaims the bandwidth of the tasks in Inactive state.
  It does so by decrementing the runtime of the executing task Ti at a pace equal
  to
 
-           dq = -max{ Ui / Umax, (1 - Uinact - Uextra) } dt
+           dq = -(max{ Ui, (Umax - Uinact - Uextra) } / Umax) dt
 
  where: