R1:b81c8380fb97

Document support for "limit" in tokenizer-based Custom Fields

Summary:
Fixes T13356. This option is supported and works fine, it just isn't documented.

Add documentation and fix the config option to actually link to it to make life a little easier.

Test Plan: Read documentation.

Maniphest Tasks: T13356

Differential Revision: https://secure.phabricator.com/D20691