Ah, right, actual codegen.
I'd personally start with looking for include! macro usage to see if the crate uses codegen and then just reset to the conservative behavior then. The other possible behavior would be just to do the scrape after the default target build and scrape $OUT_DIR as well.
It's definitely also worth mentioning that macros can "obfuscate" a #[cfg] so it wouldn't be found by the scrape. Thankfully, the big cfg macro, cfg_if!, is caught by the scrape, but other macros easily might not contain the literal #[cfg syntax that the scrape is looking for.
I'm sort of brainstorming from the direction of docs really wants to build only one platform by default, and any smarts beyond that are attempts to make the behavior more likely to generate docs for the meaningful targets.
The docs infra would prefer to underestimate the required targets, the empathy towards users would prefer to overestimate the required targets.
The only "safe" behavior is to continue cross-documenting all tier-1 targets; any other solution is going to miss some edge cases (fairly trivially true due to halting problem for the build script). So it feels to me like the only "good" solution is to drop the default number of builds and have some small number of fairly simple rules as to when tier-1 doc builds for other targets are implicitly enabled.