ANDROID: sched: Clean-up SchedTune documentation

SchedTune's documentation file hasn't been updated in a while and
occasionally contains stale informations in a few places.

Clean-up the doc file by:
 - removing the references to the now defunct sched-freq;
 - replacing references to SCHED_LOAD_SCALE by SCHED_CAPACITY_SCALE;
 - removing the statement that the wake-up path is not boost-aware;
 - removing reference to negative boosting;
 - removing 'motivation' paragraphs that aren't really relevant anymore;
 - and making sure to fit all the text in 80 chars.

No fundamental changes about the core of the explanations.

Bug: 120440300
Fixes: 159c14f0 ("ANDROID: sched: fair/tune: Add schedtune with
cgroups interface")
Change-Id: I8ad92a93082e2efe92bc3a7526960e50032be909
Signed-off-by: Quentin Perret <quentin.perret@arm.com>