Document rsync as a server-side prerequisite for media store import

The import-synapse-media-store task synchronizes the media store with
ansible.posix.synchronize delegated to the server itself, which requires
the rsync binary on the server. Nothing in the playbook installs it, so
the import failed on minimal systems with "Failed to find required
executable rsync". Document the prerequisite.

Fixes #2551

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/importing-synapse-media-store.md

@@ -13,6 +13,8 @@ You can manually import your `media_store` files from a previous installation of
 
 Before doing the actual data restore, **you need to upload your media store directory to the server** (any path is okay).
 
+You also need the [rsync](https://rsync.samba.org/) utility installed **on the server**, as the import performs a server-side `rsync` synchronization. The playbook does not install it for you. On most distributions, it is available as a package called `rsync`.
+
 If you are [storing Matrix media files on Amazon S3](configuring-playbook-s3.md) (optional), restoring with this tool is not possible right now.
 
 As an alternative, you can perform a manual restore using the [AWS CLI tool](https://aws.amazon.com/cli/) (e.g. `aws s3 sync /path/to/server/media_store/. s3://name-of-bucket/`)