fix(cpp): model bdlbb::BlobBuffer::buffer and trim model comments

Address review feedback on the bdlbb::Blob models:

- Add a summary for BlobBuffer::buffer(), which returns the
  bsl::shared_ptr<char> that owns the bytes. No shared_ptr rows are
  needed: SmartPointer.qll already covers bsl::shared_ptr::get().
- Add a harness case that reads through blob.buffer(0).buffer().get().
- Shorten the section comments in bdlbb.model.yml and the change note.