Improve Conduit type handling for *.edit endpoints

Summary:
Ref T9964. Three goals here:

• Make it easier to supply Conduit documentation.
• Make automatic documentation for *.edit endpoints more complete, particularly for custom fields.
• Allow type resolution via Conduit types, so you can pass ["alincoln"] to "subscribers" instead of needing to use PHIDs.

Test Plan:

• Viewed and used all search and edit endpoints, including custom fields.
• Used parameter type resolution to set subscribers to user "dog" instead of "PHID-USER-whatever".
• Viewed HTTP parameter documentation.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9964

Differential Revision: https://secure.phabricator.com/D14796