Mark installed doc files as %doc
    
    I'm keeping this separate becuase it's logically a seperate change.
    In the -devel package, we installed the files once. I think it's nice
    to mark them as %doc, so that it shows up correctly when filtered
    by rpm attributes.
    
    After a mock build with the changes here:
    
    $ rpm -Vv -p /var/lib/mock/fedora-rawhide-x86_64/result/rust-rust-ini-devel-0.17.0-6.fc37.noarch.rpm
    ...
    missing     /usr/share/cargo/registry/rust-ini-0.17.0
    missing     /usr/share/cargo/registry/rust-ini-0.17.0/.cargo-checksum.json
    missing     /usr/share/cargo/registry/rust-ini-0.17.0/Cargo.toml
    missing   l /usr/share/cargo/registry/rust-ini-0.17.0/LICENSE
    missing   d /usr/share/cargo/registry/rust-ini-0.17.0/README.rst
    missing     /usr/share/cargo/registry/rust-ini-0.17.0/examples
    missing     /usr/share/cargo/registry/rust-ini-0.17.0/examples/test.rs
    missing     /usr/share/cargo/registry/rust-ini-0.17.0/rustfmt.toml
    missing     /usr/share/cargo/registry/rust-ini-0.17.0/src
    missing     /usr/share/cargo/registry/rust-ini-0.17.0/src/lib.rs