Bypassing the scheduler during a live migration or evacuation
is not something we want people to be doing as it can easily fail
if the specified host is already full or doesn't provide something
required by the instance, plus it's a nightmare for tracking allocations
in the Placement service when we're bypassing the scheduler.
Because of this, we should have some warnings in the API reference
about doing this, which this patch adds.
Change-Id: I85e7c2677f4d5eccc1e7f349de06960b53ef148d
a4609a834b