docs: add retention policy notes

docs/sources/configuration/direct_db_datasource.md

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+# SQL Data Source Configuration
+
+## MySQL
+
+In order to use _Direct DB Connection_ feature you should configure SQL data source first.
+
+![Configure MySQL data source](../img/installation-mysql_ds_config.png)
+
+Select _MySQL_ data source type and provide your database host address and port (3306 is default for MySQL). Fill
+database name (usually, `zabbix`) and specify credentials.
+
+### Security notes
+
+As you can see in _User Permission_ note, Grafana doesn't restrict any queries to the database. So you should be careful
+and create a special user with limited access to Zabbix database. Grafana-Zabbix plugin uses only `SELECT` queries to
+`history`, `history_uint`, `trends` and `trends_uint` tables. So it's reasonable to grant only SELECT privileges to
+these tables for grafana user. But if you want to use this MySQL data source for querying another data, you can
+grant SELECT privileges to entire zabbix database.
+
+Also, all queries are invoked by grafana-server, so you can restrict connection to only grafana host.
+
+```sql
+GRANT SELECT ON zabbix.* TO 'grafana'@'grafana-host' identified by 'password';
+```
+
+## PostgreSQL
+
+Select _PostgreSQL_ data source type and provide your database host address and port (5432 is default). Fill
+database name (usually, `zabbix`) and specify credentials.
+
+![Configure PostgreSQL data source](../img/installation-postgres_ds_config.png)
+### Security notes
+
+Make sure you use read-only user for Zabbix database.
+
+## InfluxDB
+
+Select _InfluxDB_ data source type and provide your InfluxDB instance host address and port (8086 is default). Fill
+database name you configured in the [effluence](https://github.com/i-ky/effluence) module config (usually, `zabbix`) and specify credentials.
+
+![Configure InfluxDB data source](../img/configuration-influxdb_ds_config.png)