rustdoc CURRENT_RUSTC_VERSION shows incorrect version between version bump and stage0 bump

[ehuss]

Items marked with the stability attribute and CURRENT_RUSTC_VERSION will show the wrong version in nightly rustdoc output in between the version bump and the stage0 bump. That means in this sequence:

1. Item gets marked with CURRENT_RUSTC_VERSION. Example, on nightly the current rust version is 1.80, which is what rustdoc shows.
2. Version is bumped example bumps to 1.81.
3. Nightly docs for CURRENT_RUSTC_VERSION marked items now show 1.81 when they should say 1.80.
4. Stage0 is bumped, and CURRENT_RUSTC_VERSION placeholders get replaced example. The item is now correctly marked 1.80.

This can be confusing since there can be a window of several days where the wrong version is displayed.