Add register_custom_normalizer function and fix error message

[JamesHWade]

This PR makes two small changes:

1. Fixes the incorrect error message that directs users to a non-existent function path for registering custom normalizers
2. Adds a new register_custom_normalizer function with documentation and examples

Fixes #1935

[Copilot]

Pull Request Overview

This PR fixes an incorrect error message and adds a new function to register custom normalizers.

• Updates error message to reference the new registration function
• Introduces register_custom_normalizer with documentation and examples

[cpsievert]

Thanks!