Boost: add docs for filters in the Cache module

[donnchawp]

Add documentation to Cache_API.md for the filters used in this part of the plugin.

Also see #36209

Proposed changes:

• add docs to Cache_API.md

Other information:

• [ ] Have you written new tests for your changes, if applicable?
• [ ] Have you checked the E2E test CI results, and verified that your changes do not break them?
• [ ] Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

No testing possible. Just documentation