Clean up wasi-libc module doc comment

(cherry picked from commit 9ac4a6b)

src/wasi/mod.rs

@@ -1,5 +1,7 @@
-// [wasi-libc](https://github.com/WebAssembly/wasi-libc) definitions.
-// `wasi-libc` project provides multiple libraries including emulated features, but we list only basic features with `libc.a` here.
+//! [wasi-libc](https://github.com/WebAssembly/wasi-libc) definitions.
+//!
+//! `wasi-libc` project provides multiple libraries including emulated features, but we list only
+//! basic features with `libc.a` here.
 
 use core::iter::Iterator;