pgbackrest: document technical debt and other details

the comment at the top of the playbook section will hopefully help us to
remember to keep the playbook URLs in order if we rename the section.

howto/postgresql.md

@@ -1838,6 +1838,11 @@ encountered such errors so far.
 
 ### Backups misconfigurations
 
+<!-- note that this section is cross-referenced from the -->
+<!-- PgBackRestStanzaError, PgBackRestRepositoryError and -->
+<!-- PgBackRestExporterFailure alerts in prometheus-alerts.git change the -->
+<!-- link target there if you change the heading here. -->
+
 A certain number of conditions can be raised by the [backups
 monitoring system](#backups-monitoring) that will raise an alert. Those are, at the time
 of writing:
@@ -1861,6 +1866,10 @@ In all case, another idea is to [check backup health](#checking-backup-health).
 confirm (or not) that stanzas are properly configured, and outline
 misconfigured stanza or errors in the global repository configuration.
 
+The status code 99 means "other". This generally means that some external reason
+is causing things to not run correctly. For example permission errors that make
+the exporter unable to read from the backup directories.
+
 ### Disk is full or nearly full
 
 It's possible that pgBackRest backups are taking up all disk space on
@@ -2797,6 +2806,13 @@ See also:
 
 -->
 
+Technical debt that needs to eventually be addressed:
+
+* the `pgbackrest_exporter` currently runs as root since it needs to be able to
+  read from backup directories under all of the backup users. We want to
+  implement a better method for the exporter to get access to the files without
+  running as root.
+
 ## Goals
 
 <!-- include bugs to be fixed -->