Add section about async loops to control-flow and/or async/await page(s)

As there are not all syntactical gimmicks from JS available in ReScript (like `for-await...of`), it would help to cover these special cases in their own section. One possible workaround is described here: https://github.com/rescript-association/rescript-core/blob/main/src/Core__AsyncIterator.resi#L35