Specifically describe "local.json" in backup/migration config documentation

Summary: Fixes T7648. The existence of this file is not necessarily obvious and is important to backup, transfer, and restore.

Test Plan: Read documentation.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T7648

Differential Revision: https://secure.phabricator.com/D12183