As of the most recent Rust nightly rustup no longer installs documentation by default. If you already have docs installed then future toolchain upgrades will keep them installed, but fresh installs will not receive docs. We’ve done this because the docs have significant download size and are not critical for most developers to possess locally.
If you do want docs for your active toolchain you can run:
rustup component add rust-docs
To remove docs from your existing, updated, nightly install, run:
rustup component remove rust-docs