Clarify to users what objects may be cached, and highlight the option used to cache default non-serializable data.

Purpose: Improving new-to-Rails users' experience, as this detail may not be obvious, costing them time and effort spent debugging.

Co-authored-by: Hartley McGuire <skipkayhil@gmail.com>
Co-authored-by: Jonathan Hefner <jonathan@hefner.pro>
(cherry picked from commit de12c18a)