Enforce in bootstrap that doc must have stage at least 1

[Kobzol]

Following with the bootstrap cleanups, this time around doc steps. Should be pretty straightforward, because the supporting infrastructure was already there.

The only thing I found a bit fishy is using Mode::ToolBootstrap as a "catch-all" mode for non-rustc-private steps in tool_doc!, but I don't think that we need to distinguish the tools in some special way when documenting them, apart from supporting rustc_private.

Before, x doc more or less defaulted to what we call stage 2 now. Now it is properly stage 1, so e.g. x doc compiler documents the compiler using the stage0/beta rust(do)c.

r? @jieyouxu

try-job: dist-aarch64-msvc