Improve Git SSH usage documentation

The documentation regarding SSH is misleading.
If I'm using SSH and the private key is associated with a user (common use-case, i.e. the private key is in `~/.ssh/id_rsa`), then I should **not** use the `identity` parameter, as any other user than the one who has the key may access it, but use the `user` parameter to trigger an `su`.

This changeset aims to make this clearer.