R1:5de10185ce5b

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

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