Document the need to purge caches after updating differential.generated-paths

Summary: Fixes T6378.

Test Plan: Set config to /.*/, created a new diff, everything was collapsed as generated.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T6378

Differential Revision: https://secure.phabricator.com/D12159