docs(turbopack): Document build dependency on clang for rocksdb (#72493)
Noticed this after rebuilding on top of #71688
Most Linux distributions default to `gcc` for their default `cc`
implementation (e.g. via Debian's `build-essential` meta-package, which
we can probably implicitly assume is installed), which means that they
won't typically have `clang` installed.
`librocksdb-sys` seems to always depend on `clang` on Linux. It looks
like this might be a consequence of using
[rust-bindgen](https://github.com/rust-lang/rust-bindgen) (though I'm
confused why this didn't come up earlier), as it looks like rocksdb can
otherwise build with gcc.
This shouldn't be an issue for macos, since `clang` is the default `cc`
on that platform.