Add DebugFlag BackfillMap document

Update slurm.conf man page for DebugFlag BackfillMap. This should be
considered part of commit 3c2bffb6

doc/man/man5/slurm.conf.5

@@ -436,6 +436,11 @@ Valid subsystems available today (with more to come) include:
 \fBBackfill\fR
 Backfill scheduler details
 .TP
+\fBBackfillMap\fR
+Backfill scheduler to log a very verbose map of reserved resources through
+time. Combine with \fBBackfill\fR for a verbose and complete view of the
+backfill scheduler's work.
+.TP
 \fBBGBlockAlgo\fR
 BlueGene block selection details
 .TP