Small documentation update for CodeGeneration for Durable Functions

[KeesV]

Small update to the documentation for CodeGeneration for Durable Functions

• Remove instruction to mark the reference with specific attributes, as these attributes only make sense when adding a Project Reference, and we're talking about a PackageReference here
• Add <EmitCompilerGeneratedFiles> to make sure generated code is actually output somewhere

Pull request checklist

• My changes do not require documentation changes
  • Otherwise: Documentation PR is ready to merge and referenced in pending_docs.md
• My changes should not be added to the release notes for the next release
  • Otherwise: I've added my notes to release_notes.md
• My changes do not need to be backported to a previous version
  • Otherwise: Backport tracked by issue/PR #issue_or_pr
• I have added all required tests (Unit tests, E2E tests)

[ConnorMcMahon]

Looks great to me. Thanks for these changes, a lot of tweaks were made at the last minute to this project so I'm not surprised there were small gaps like these. Thanks for your contributions!