Add documentation for customizing fonts

Summary: Fixes T7888, adds clarification of property and an example.

Test Plan: Read the text, copy/paste example.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7888

Differential Revision: https://secure.phabricator.com/D12508