Improve messaging and documentation around surplus schemata

Summary: Fixes T6795. Fixes T6813. We can give more tailored instructions for surplus schemata than we currently do, and provide more information on resolving them.

Test Plan:
  - Ran `storage adjust` with just surplus schemata (friendly warning).
  - Ran `storage adjust` with surplus schemata and other serious errors (more severe error).
  - Read document.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T6795, T6813

Differential Revision: https://secure.phabricator.com/D11054