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>
One thing I see that could be confusing is that you say there are 3 priorities but then describe the second one as being a function of the number of missing replicas.  From the code change there is priority 1, priority 200 + missing replicas, priority 255 for log based recovery.  I would call those 3 priority types not 3 priorities since they take on values from 1 to 255.