factor the docs out as well

crates/wasi/src/lib.rs

@@ -9,7 +9,16 @@ pub use wasi_common::{WasiCtx, WasiCtxBuilder};
 wasmtime_wiggle::define_struct_for_wiggle!({
     witx: ["../wasi-common/WASI/phases/snapshot/witx/wasi_snapshot_preview1.witx"],
     ctx: WasiCtx,
-    instance: Wasi,
+    instance: {
+        name: Wasi,
+        docs: "An instantiated instance of the wasi exports.
+
+This represents a wasi module which can be used to instantiate other
+wasm modules. This structure exports all that various fields of the
+wasi instance as fields which can be used to implement your own
+instantiation logic, if necessary. Additionally [`Wasi::get_export`]
+can be used to do name-based resolution."
+    }
 });
 
 pub fn is_wasi_module(name: &str) -> bool {