External blocks provide declarations of items that are not defined in the current crate and are the basis of Rust's foreign function interface. These are akin to unchecked imports.
There's no point in exposing documentation in your codebase on these. It's not coming from your code.
rustdocflags is not a valid Cargo.toml key. You need to invoke cargo doc --document-private-items --open or set the RUSTDOCFLAGS environment variable, or modify .cargo/config.