Config User Extra Fields: link to the doc (like Maniphest)

Summary:
Add a documentation link in a specific Config page, that is:

Config > Setting > user.custom-field-definitions

Test Plan:
Visit the page and click on the new link:

• /config/edit/user.custom-field-definitions/

Visit this page that was the inspirational page:

• /config/edit/maniphest.custom-field-definitions/

No nuclear implosions.

Reviewers: O1 Blessed Committers, aklapper

Reviewed By: O1 Blessed Committers, aklapper

Subscribers: aklapper, tobiaswiese, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25507