Update docs for index enhancements

[andyw8]

Due to #2704, we need to update the add-ons docs to mention the code units cache.

Changes were made in #2857 which aren't yet reflected in the docs.

[github-actions]

This issue is being marked as stale because there was no activity in the last 2 months

[vinistock]

Is this still relevant now that we changed the API for add-ons to use the declaration listener? They no longer need to pass the code units cache, it's all abstracted away.

[andyw8]

That's true, but the docs still need some updates, e.g. they mention register_enhancement which was removed in #2857. I've updated the issue description.