Document top-level wasi functions for wasi-clocks

[ricochet]

I expected this proposal to include wasi::clock_time_get as used by rust wasi/time.rs. A little bit of searching and I found the source for this here: https://github.com/bytecodealliance/preview2-prototyping/blob/7356e57599c869772dd12ca59879d0cba826a320/src/lib.rs#L307-L331