Add some documentation about how to set paths with owners.edit

Summary:
Ref T9964.

• New mechanism for rich documentation on unusual/complicated edits.
• Add some docs to paths.set since it's not self-evident what you're supposed to pass in.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9964

Differential Revision: https://secure.phabricator.com/D14791