Consider allowing to build docs for the current kernel config #447

Initially, we had the rustdoc target build customized docs for the current kernel configuration. Then, in #446 (suggested in #442) we started using doc_cfg and the intention is that we build all the docs.

However, it could perhaps be useful to still allow users to build a customized version of the docs for the kernel configuration, e.g. for local development (i.e. faster generation) or for distributions that wish to provide trimmed down docs for the kernels they release. See discussion at #446 for details.

For the moment, let's keep things simple. If somebody needs the feature, it is easy to implement (although it may require some churn to move to a doc_all gate instead of the doc one).