Support for rustdoc --check was added in #78984, and it can be seen in the output from rustdoc --help:

$ rustdoc --help
...
        --check         Run rustdoc checks

However there is no mention of it on:
https://doc.rust-lang.org/rustdoc/command-line-arguments.html

Which is built from:
https://github.com/rust-lang/rust/blob/e4b1d5841494d6eb7f4944c91a057e16b0f0a9ea/src/doc/rustdoc/src/command-line-arguments.md