scratch that. that note already exists in sql-notify.html. Instead, I'd modify that section to note that you can check queue usage with your new function.
I have already done so. Under the paragraph about the queue filling up, I have added:
"The function <function>pg_notify_queue_saturation</function> returns the proportion of the queue that is currently occupied by pending notifications."
A link from here back to the section in System Information Functions might be sensible?
I will rename the function with _usage as you suggest, and add the explanation of the return value in the docs.
