Add comment to ReactiveCaching module of new docs

What does this MR do?

The ReactiveCaching module previously had a lengthy documentation comment. The content of the comment was added to our utilities documentation in !19129 (merged).

This change updates the comment above the module to refer to the new documentation page.

Issue: #35748 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Closes #35748 (closed)