Update cache integration section

[letitz]

The cache integration section does not reflect what's now implemented in Chromium as a result of rollout experience.

Broadly, cached resources are now subject to PNA checks. There are subtleties to outline:

• what happens for partially-cached resources and range requests
• cache entry invalidation
• interaction with secure contexts vs non-secure contexts

[valenting]

Hi @letitz any progress on this one? https://wicg.github.io/private-network-access/#http-cache-subresources is quite empty, while https://wicg.github.io/private-network-access/#http-cache-security just states that the section is outdated.

I'd appreciate even a brief outline of what Chrome is currently doing. Thanks!