Documentation to quickly check `--trace-on` implementations

Having to check the source in order to remember:
1. What `--trace-on` arguments are available.
2. What each one actually does
eats into developer time.

So, a modest proposal:

- [ ] Define a `# TRACEON` comment forma. Just a prefix probably, multiple consecutive lines comprising a single entry.
- [x] Python script utilising AST to scan all the source for `logger.trace_if(...)` invocations, and the attached comments. It should then output the results to a file in some format that ....
- [x] `--help trace-on`, or if needs be `--trace-on-help` will then consult that file and output the list and documentation. Can possibly also make it only output the relevant information for a given possibility.