Change missing_docs from deny to warn.

[Dirbaio]

It is annoying to get hard errors while developing, it prevents testing things out without documenting them because it fails the build.

This changes it to warn, which has the same effect in CI (because it already has RUSTFLAGS: '--deny warnings'), but still allows buildilng.

[rust-highfive]

r? @eldruin

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[bors[bot]]

Build succeeded:

• ci-linux (1.54.0, x86_64-unknown-linux-gnu)
• ci-linux (stable, thumbv6m-none-eabi)
• ci-linux (stable, thumbv7m-none-eabi)
• ci-linux (stable, x86_64-unknown-linux-gnu)
• ci-linux-test (1.54.0, x86_64-unknown-linux-gnu)
• ci-linux-test (stable)
• fmt