os_target and arch_target have no default value

Hi!

There is a little [reference](https://docs.conan.io/en/latest/systems_cross_building/cross_building.html#conan-settings) about `settings.os_target` and `settings.arch_target`, however, we have the issues https://github.com/conan-io/conan/issues/4519 https://github.com/conan-io/conan/issues/2428 https://github.com/conan-io/conan/issues/2301 that are related to `targets`.

I think we should consider a better explanation about them, like a real use case, and most important, document that they don't have a default value, that means, if you declare one of targets, you will need to evaluate manually, as commented at https://github.com/conan-io/conan/issues/4519

Regards!