Recapitulate type aliases in documentation

[itowlson]

The 0.3.0 draft of monotonic-clock adds a new wait-for API which takes a duration:

https://github.com/WebAssembly/wasi-clocks/blob/646092fb6428b2ecf1b97452a71a44ec3f2fd3ce/wit-0.3.0-draft/monotonic-clock.wit#L42-L44

Unfortunately the Rust bindings (and I'm sure they're not alone) don't surface the duration alias, making the documentation cryptic:

When using primitive type aliases, therefore, it would be useful to recapitulate their interpretation at the point of use.