Documentation

After reading the source code in gc.c and the garbage collector documentation in garbage_collector.md, I wondered why most objects turned out to be reachable.
I found an old thread on the Python-Dev mailing list, and I think it would be useful to add a link to it in the documentation and source code.

@tim-one as an author of Some dull gc stats
@pablogsal and @nascheme as GC experts

Linked PRs

• gh-136693: Add link to original post about moving unreachable objects in GC #136694