os_target and arch_target have no default value

[uilianries]

Hi!

There is a little reference about settings.os_target and settings.arch_target, however, we have the issues conan-io/conan#4519 conan-io/conan#2428 conan-io/conan#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 conan-io/conan#4519

Regards!