checkbackuppg error that was happening today but was not documented

I'm not 100% certain about what to do in the case that the archive is
valid, so I indicated that maybe we want to extract it in a subdir. we
can adjust that when we find a case where that happens. today's case was
a corrupted archive that we could delete.

howto/postgresql.md

@@ -2095,6 +2095,16 @@ Alternatively, a dump/restore will reset the timeline to the normal
 "1", but then you'd need to move the directory out of the way and make
 a new full backup.
 
+### NOT-A-DIR
+
+The `dsa-check-backuppg` script expects to find only subdirectories to the
+`rootdir`, e.g. `/srv/backups/pg`. This error indicates that there is a file
+present in the rootdir that is not a directory.
+
+The file might need to be uncompressed into a subdirectory if possible, or
+removed if the archive is corrupted for example if a backup got stopped midway
+through to completion.
+
 ## Disaster recovery
 
 If a PostgreSQL server is destroyed completely or in part, we need to