Document `rust_fuzzer_test_input`
Document `rust_fuzzer_test_input`

Adds a dummy documentation for `rust_fuzzer_test_input`.

When passing `-Fmissing_docs` to cargo CLI, `libfuzzer-sys` won't compile because of the non-documented public auto-generated function.
rust-fuzz/libfuzzer,rust-fuzz/libfuzzer,rust-fuzz/libfuzzer,rust-fuzz/libfuzzer,rust-fuzz/libfuzzer