doc: updates on Backfill Reservation

The logic was changed by:

0985ae71bce32c4d9e0e9e9f68bed38eb3c26897
osd: prioritize backfill based on *how* degraded

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
Merge pull request #2736 from dachary/wip-backfill-documentation

doc: updates on Backfill Reservation

Reviewed-by: David Zafman <david.zafman@inktank.com>
Reviewed-by: Dan Mick <dan.mick@inktank.com>