bootstrap: rustdoc is off-by-1-stage

[RalfJung]

The Step that builds rustdoc decrements the stage by 1 before building. That's very confusing, it means ./x.py build rustdoc --stage 0 is a NOP even though usually stage 0 is the stage you want to build!

I think instead Builder::rustdoc should do this decrementing. Or maybe there should be an AssembleRustdoc step that matches the current Assemble step and does the same stage decrementing there.

The current situation is also inconsistent between rustdoc and other similar tools like clippy or Miri; only rustdoc does the stage decrementing in its build Step.

[onur-ozkan]

#119946 :(