Make generate_function_docs.sh error if there is a function without documentation

[alamb]

Is your feature request related to a problem or challenge?

Part of https://github.com/apache/datafusion/issues/12740

All functions in DataFusion should be documented on the documentation site https://datafusion.apache.org/user-guide/sql/index.html

Right now, we rely on manual code reviews to ensure this is the case, but we don't have any way to ensure all functions are documented (and indeed some are not)

Describe the solution you'd like

I would like CI to fail if a new function is added without documentation

Describe alternatives you've considered

I think we can modify the ./dev/update_function_docs.sh scrip / function to error when functions do not have documentation

Additional context

We do have to wait for all content to be ported to code before we can enable this check

[alamb]

Here is the PR to make it error: https://github.com/apache/datafusion/pull/12938