Allow user code to access XML document comments generated by the source generator in custom transformer

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Is your feature request related to a problem? Please describe the problem.

Custom transformer can modify api document, but only generated transformer can access XML document comments cache objects. I hope I can use document comments like generated transformer.

### Describe the solution you'd like

Make XML document comments cache objects become a singlation service.

### Additional context

_No response_

Start by tracing how the generated transformer accesses the XML document comments cache and how custom transformers receive the API document. Determine the service lifetime and access point needed to expose the same cache to user code. Done means a custom transformer can access the XML document comments without relying on the generated transformer.

Written by the indexing model from the issue text.