When setup issues raise opcache configuration errors, point at the opcache configuration page

Summary:
Fixes T11746. The opcache docs are on a different page, so point there if we're raising opcache issues.

(It's possible for a setup issue to say "configure X, or configure Y", where X is opcache and Y is non-opcache, so we may want to render both links.)

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11746

Differential Revision: https://secure.phabricator.com/D16685