Recapitulate type aliases in documentation

[itowlson]

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

wasi-clocks/wit-0.3.0-draft/monotonic-clock.wit

Lines 42 to 44 in 646092f

	wait-for: func(
	how-long: duration,
	);

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.