Figure out how to document anonymous structs/unions

[leofang]

Ex: https://nvidia.github.io/cuda-python/module/driver.html#cuda.bindings.driver.CUasyncNotificationInfo

The member info is of type anon_union2, which is undocumented.

Setting this to P2 without a milestone, as people can always refer to the C API reference. But we don't want to lose track of this.