Add note about changing JobAcctGatherType in slurm.conf
sched/backfill - Prevent invalid memory reference

Induced by bf_continue option and deleting a partition.
Minor document update to include note about PrivateData=Usage for the
slurm.conf when using the DBD.
minor capitalization of words
Expand information reported with DebugFlags=backfill

Previous logic only reported un-reserved node map.
New logging adds information about each job testing and where/when
it is scheduled resources.
Merge branch 'slurm-2.6'