Add better instructions for creating `execute` as batched

[benjie]

Essentially:

1. Map over the input values to turn them into something useful for step 2.
2. Do a single await to do your actual work. NOT IN A LOOP.
3. Map over the input values again, for each one returning the relevant data from step 2 that matches this input value.

Essentially this is the same strategy as DataLoader except Grafast steps are more powerful and Step::execute() has access to a lot more context.