cargo doc: pass --cap-lints allow when documenting dependencies #5749
Description
When rust-lang/rust#52354 lands, rustdoc will be able to accept --cap-lints to suppress lints when building documentation. Cargo should take advantage of this to pass --cap-lints allow when documenting dependencies, like how cargo build does the same. This will help contain the damage of rust-lang/rust#51468 and errors like it.