What with it still not being public at the time, I didn’t think of it in regard to this question, but @OtisTDog 's Semantics of Standard Rules Actions in Inform 7 6M62 is something like the reference @Jim_Aikin was asking for here, at least for the action rules. I think it doesn’t exactly hit the presentation/level of granularity that Jim was seeking, given my reading of Jim’s wishes, but it comes closer than anything else.
It also recently occurred to me to enthusiastically recommend “Background Activities”, pp. 60-66 of Ron Newcomb’s Inform 7 for Programmers (PDF). It’s got a great description of several things including how room descriptions work that deserve a wider audience than people who hadn’t been scared off by that document’s technical tone way before p. 60.
I don’t think either would much help with the describe what’s on scenery supporters in room descriptions rule per se, but they’re darn useful.