fix: rephrase the Postgres PGDATA documentation and add a warning (#2340)

Signed-off-by: Matthias Riegler <[email protected]>

postgres/content.md

@@ -96,7 +96,9 @@ See the PostgreSQL documentation on [`pg_hba.conf`](https://www.postgresql.org/d
 
 ### `PGDATA`
 
-This optional variable can be used to define another location - like a subdirectory - for the database files. The default is `/var/lib/postgresql/data`. If the data volume you're using is a filesystem mountpoint (like with GCE persistent disks) or remote folder that cannot be chowned to the `postgres` user (like some NFS mounts), Postgres `initdb` recommends a subdirectory be created to contain the data.
+> **Important Note:** when mounting a volume to `/var/lib/posgresql`, the `/var/lib/postgresql/data` path is a local volume from the container runtime, thus data is not persisted on the mounted volume.
+
+This optional variable can be used to define another location - like a subdirectory - for the database files. The default is `/var/lib/postgresql/data`. If the data volume you're using is a filesystem mountpoint (like with GCE persistent disks), or remote folder that cannot be chowned to the `postgres` user (like some NFS mounts), or contains folders/files (e.g. `lost+found`), Postgres `initdb` requires a subdirectory to be created within the mountpoint to contain the data.
 
 For example: