Document user batch changes credentials aren't used for importing changesets

Created by: eseliger

Started conversation in https://github.com/sourcegraph/customer/issues/378. In short: we don't clearly document that importing and syncing changesets requires a batch changes site credential token, and doesn't use user tokens.

Also make sure to add it to this page: https://docs.sourcegraph.com/batch_changes/how-tos/tracking_existing_changesets

See also: #21889 (closed); ideally we should address both together.