Expand backup controller documentation

doc/man/man5/slurm.conf.5

@@ -303,6 +303,9 @@ to a "standby" mode when the ControlMachine becomes available once again.
 This should be a node name without the full domain name.   I.e., the hostname
 returned by the \fIgethostname()\fR function cut at the first dot (e.g. use
 "tux001" rather than "tux001.my.com").
+The backup controller recovers state information from the
+\fBStateSaveLocation\fR directory, which must be readable and writable from both
+the primary and backup controllers.
 While not essential, it is recommended that you specify a backup controller.
 See  the \fBRELOCATING CONTROLLERS\fR section if you change this.