Check if new notification events are documented

[zzak]

Similar to check-config-docs, I think it would be nice if we made sure new notifications were documented.

For example:
rails/rails#46603 (comment)

[skipkayhil]

This one may be tough to get high coverage on. Using my current method of AST parsing will be tough because there is so much variety in how hooks are defined. The ideal case looks something like this:

ActiveSupport::Notifications.instrument("process_action.action_dispatch", payload)

because we can easily extract the string, but in reality we have

• events defined as variables https://github.com/rails/rails/blob/c31684294f7c86f08f943b5b7fe8c10793e49f1a/activerecord/lib/active_record/core.rb#L232
• events defined in constants https://github.com/rails/rails/blob/c31684294f7c86f08f943b5b7fe8c10793e49f1a/actionpack/lib/action_dispatch/middleware/stack.rb#L49
• dynamically generated event names https://github.com/rails/rails/blob/c31684294f7c86f08f943b5b7fe8c10793e49f1a/activestorage/lib/active_storage/previewer.rb#L68
• etc.

I'll push up a script to get all of the easy ones once I refine it a bit more, but this may end up requiring a new approach 🤔