Documentation for cache-related proxy filters

## What is the problem your feature solves, or the need it fulfills?

Currently there's no documentation for the cache-related filters in `pingora-proxy`, e.g. in the [`phases.md`](https://github.com/cloudflare/pingora/blob/main/docs/user_guide/phase.md) docs.

## Describe the solution you'd like

We should add the corresponding descriptions for phases such as `request_cache_filter`.