Merge pull request #1092 from wol-win/server_migration_doc_update

Server migration doc update

docs/maintenance-migrating.md

@@ -1,3 +1,7 @@
+> **Note**: This migration guide is applicable if you migrate from one server to another server having the same CPU architecture (e.g. both servers being `amd64`). 
+> 
+> If you're trying to migrate between different architectures (e.g. `amd64` --> `arm64`), simply copying the complete `/matrix` directory is not possible as it would move the raw PostgreSQL data between different architectures. In this specific case, you can use the guide below as a reference, but you would also need to dump the database on your current server and import it properly on the new server. See our [Backing up PostgreSQL](maintenance-postgres.md#backing-up-postgresql) docs for help with PostgreSQL backup/restore.
+
 # Migrating to new server
 
 1. Prepare by lowering DNS TTL for your domains (`matrix.DOMAIN`, etc.), so that DNS record changes (step 4 below) would happen faster, leading to less downtime