Update http_cache.rst

Explained how the parameters are passed to the controller - named parameters
Clarifying a bit more that you're passing variables into your controller
minor #4338 ESI Variable Details Continuation (Farkie, weaverryan)

This PR was merged into the 2.3 branch.

Discussion
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ESI Variable Details Continuation

This continues #4098.

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | n/a

Thanks!

Commits
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ded7577 Clarifying a bit more that you're passing variables into your controller
5877d02 Update http_cache.rst