Http cache validation rewording

Hiya guys!

This comes after #3596.

I realized that the sentence on the notes was a bid hard to read. I also removed a usage of
the word "client" right before this, so I can use the word "cache" instead (meaning, I want
the user to be thinking about the reverse proxy, not a user's browser cache - that's valid,
but not the typical use-case).

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | -

Thanks!