Synchronize: Document the private_key option (#32754)

`synchronize` has supported the `private_key` option for a long time,
apparently. But for some reason it was never documented.

Today I managed to workaround the synchronize quoting bug by just using

```
private_key: /path/to/id_rsa
```

instead of

```
rsync_opts:
  - "--rsh 'ssh -i /path/to/id_rsa'"
```

So, I'll just go ahead and document this useful option ...

lib/ansible/modules/files/synchronize.py

@@ -146,6 +146,10 @@ options:
     type: bool
     default: 'no'
     version_added: "2.0"
+  private_key:
+    description:
+      - Specify the private key to use for SSH-based rsync connections (e.g. C(~/.ssh/id_rsa))
+    version_added: "1.6"
 notes:
    - rsync must be installed on both the local and remote host.
    - For the C(synchronize) module, the "local host" is the host `the synchronize task originates on`, and the "destination host" is the host