docs: Add a link to installer flag docs from backup instructions.

The documentation for restoring backups referenced that it needed to
be to the same version of PostgreSQL, but did not explain how to do
that.

Link to the relevant section of the installer documentation, and name
the flag explicitly.

Fixes: #23691

docs/production/export-and-import.md

@@ -13,7 +13,10 @@ service (or back):
     version (most precisely, one where `manage.py showmigrations` has
     the same output).
   - Backups must be restored on a server running the same PostgreSQL
-    version.
+    version. To install Zulip with the same version of PostgreSQL that
+    the backup was taken on, pass the desired version with [the
+    `--postgresql-version` argument][installer-options] when
+    installing.
   - Backups aren't useful for migrating organizations between
     self-hosting and Zulip Cloud (which may require renumbering all
     the users/messages/etc.).
@@ -56,6 +59,8 @@ service (or back):
   backend](upload-backends.md#s3-backend-configuration)
   as part of a high availability environment.
 
+[installer-options]: deployment.md#advanced-installer-options
+
 ## Backups
 
 The Zulip server has a built-in backup tool: