document pgbackrest retention period

howto/postgresql.md

@@ -2001,7 +2001,7 @@ rude, for example, see:
 
 ## Backups
 
-### DSA setup
+### Legacy
 
 We use upstream's [Continuous Archiving and Point-in-Time Recovery
 (PITR)](https://www.postgresql.org/docs/11/continuous-archiving.html) which relies on postgres's "write-ahead log" (WAL) to write
@@ -2069,6 +2069,11 @@ The new backup system is based on [pgBackRest](https://pgbackrest.org/). It work
 between the client and server and running `pgbackrest` commands, which
 encapsulates all functionality including backup, and restore.
 
+Backups are retained for 3 weeks (21 days), although the source of
+truth for this is not here but in Hiera, in `tor-puppet.git`'s
+`hiera/common/postgresql.yaml`, the
+`pgbackrest::config:global:repo1-retention-full` value.
+
 #### Puppet setup
 
 PostgreSQL servers are automatically configured to use pgBackRest to