Alternative : Detect architectures
I really like this idea! Does anyone know if there's an existing option to print cfgs seen in the code? Rustc has to detect those for macro expansion anyway so it seems like it should be possible without too much trouble.
The only potential issue is if someone tries to build for too many targets and overloads the server, but we already have a limit of 10 by default so I don't think it will be a problem.
Alternative : Single doc
While I agree this is the ideal solution, this is one of the oldest open rustdoc bugs, filed in 2012: #1998. In fact, not having this affected the design of rustdoc in many other ways; see for example #43348 and #73566. So I don't think this will happen any time soon.