`block_in_place` doesn't panic on a `spawn_local` task

[dspyz-matician]

Description

See discussion #7365

@Darksonn response indicates block_in_place on spawn_local is intended to be unsupported, and I've found (but have not been able to create MWE) that in some cases worker threads vanish permanently when engaging in this behavior. But currently it doesn't panic and the fact that it's unsupported isn't explicitly documented