docs: restructure documentation and add invalid transaction termination exception

postgresql/procedures/MANUAL.md

@@ -87,6 +87,9 @@ If you use a visual database manager, simply open each file in your query editor
 
 Partitioning requires a daily job to create new partitions for tomorrow and drop old partitions from last month.
 
+> [!WARNING]
+> **CRITICAL EXECUTION RULE**: You MUST execute the maintenance procedure as a standalone command exactly as shown below. **DO NOT** wrap it in a transaction block (`BEGIN; ... COMMIT;`) or batch it with other SQL commands in a single string, or it will crash with an `invalid transaction termination` error.
+
 If you are using **AWS RDS** or a managed database with `pg_cron` enabled, run this inside `psql`:
 
 ```sql
@@ -94,7 +97,12 @@ CREATE EXTENSION IF NOT EXISTS pg_cron;
 SELECT cron.schedule('zabbix_partition_maintenance', '30 5,23 * * *', 'CALL partitions.run_maintenance();');
 ```
 
-*(If you are self-hosting and don't have `pg_cron`, please refer to the `README.md` for instructions on setting up standard OS `cron` or systemd timers.)*
+If you are **self-hosting** and prefer standard system `cron`, simply add this to your `crontab -e`:
+
+```bash
+# Run Zabbix partition maintenance twice daily (5:30 AM and 11:30 PM)
+30 5,23 * * * psql -U postgres -d zabbix -c "CALL partitions.run_maintenance();" >> /var/log/zabbix_maintenance.log 2>&1
+```
 
 ---
 
@@ -106,7 +114,7 @@ Now that the database is fully partitioned, you can safely start Zabbix Server a
 sudo systemctl start zabbix-server
 ```
 
-*(Note: Your old history data remains in tables like `history_old`. It is no longer visible in the UI. If you need it, you must manually insert it into the new tables. See `README.md` for more details.)*
+*(Note: Your old history data remains safely preserved in tables like `history_old`. It is no longer visible in the UI, but it is available in the database if you ever need to manually migrate it.)*
 
 ---