Document just-in-time methods for handles

Summary: Fixes T7689. I'm not going to go clean up all the rest of the loadViewerHandles() calls right now since a lot of them are kind of a pain and they aren't really hurting anything so it doesn't feel very leveraged, but at least deprecate it and document the new hotness.

Test Plan:
have a look
in a book
reading rainbow

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7689

Differential Revision: https://secure.phabricator.com/D12343