Documentation: sched: Update frequency guidance explanations

Following the introduction of the top-tasks approach towards frequency
guidance, the documentation on frequency guidance needs to be updated.
Update the same with new and accurate information.

Change-Id: If7deee97c09666b7d9513f79c0e278c430acfdfd
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>