incorrect comment in bootstrap: stage 1 rustdoc *can* be used

[lolbinarycat]

The comment says that only stage 0 or stage 2 rustdoc is used, but the user can simply request stage 1 rustdoc.

rust/src/bootstrap/src/core/build_steps/tool.rs

Lines 667 to 672 in 8de4c72

	#[derive(Debug, Clone, Hash, PartialEq, Eq, Ord, PartialOrd)]
	pub struct Rustdoc {
	/// This should only ever be 0 or 2.
	/// We sometimes want to reference the "bootstrap" rustdoc, which is why this option is here.
	pub compiler: Compiler,
	}

tested by putting assert_ne!(self.compiler.stage, 1) in Rustdoc::run, then running ./x doc --stage 1 library/core, which causes the added assertion to fail.