Add an example of what an immutable cache control header might look like

[Munter]

Docs: Add example of immutable cache header to error message

When seeing the below error message about far future expiring cache headers, it's not very actionable. Add an example of how to fix the error

[alrra]

Add an example of how to fix the error

@Munter The examples are in the documentation (even how to configure some servers)

Do you feel like the documentation link is not clear enough?

---

Note: If you think something can be improved related to the docs, this is the file that should be changed.

[molant]

Maybe in this case we could make the error message clearer and have 2 different messages? One for long cache assets and another for immutable.

[molant]

BTW, the split message is tracked in #1074
Closing this PR in favor of the work in that issue.