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

[Athanasius]

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.
• [ ] 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 ....
• [ ] --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.