Update the documentation to include the `$wp_cache_mfunc_enabled` global variable.

[anton-vlasenko]

Fixes #36561

Proposed changes:

Updates the documentation to include the $wp_cache_mfunc_enabled global variable, ensuring users know it can help prevent the secret template tag from being revealed.

Other information:

• [ ] Have you written new tests for your changes, if applicable?
• [ ] Have you checked the E2E test CI results, and verified that your changes do not break them?
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Jetpack product discussion

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Testing instructions:

• Reviewed the proposed documentation change for correcteness.

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Code Coverage Summary

This PR did not change code coverage!

That could be good or bad, depending on the situation. Everything covered before, and still is? Great! Nothing was covered before? Not so great. 🤷

Full summary · PHP report

[github-actions[bot]]

This PR has been marked as stale. This happened because:

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If this PR is still useful, please do a [trunk merge or rebase](https://github.com/Automattic/jetpack/blob/trunk/docs/git-workflow.md#keeping-your-branch-up-to-date) and otherwise make sure it's up to date and has clear testing instructions. You may also want to ping possible reviewers in case they've forgotten about it. Please close this PR if you think it's not valid anymore — if you do, please add a brief explanation.

If the PR is not updated (or at least commented on) in another month, it will be automatically closed.

[github-actions[bot]]

This PR has been automatically closed as it has not been updated in some time. If you want to resume work on the PR, feel free to restore the branch and reopen the PR.