Auto merge of #50062 - varkor:xpy-check-rustdoc, r=Mark-Simulacrum

Add rustdoc to x.py check

Modifying rustc can often cause errors in rustdoc, so it's useful to include it in the steps that are checked.

One thing that I was unsure about was when to call `clear_if_dirty` (both in this step, and in other steps in relation to this one) — we want to be sure rustdoc will always be rechecked after modifying previous steps — but does this belong in rustdoc, or the other steps?

Fixes #49917.

r? @Mark-Simulacrum