[Feature Request]: Documentation about the use of MultiOutputReceiver

### What would you like to happen?

There are multiple ways to output data from a DoFn with multiple outputs:
- `DoFn.WindowedContext#output(TupleTag, T)`
- `DoFn.MultiOutputReceiver#get(TupleTag).output(T)`
Are there any reasons, other than stylistic ones, to prefer one over the other?

Generally the @ProcessElement marked method of a DoFn has a confusing number of options.

### Issue Priority

Priority: 3

### Issue Component

Component: beam-model

Start by comparing DoFn.WindowedContext#output(TupleTag, T) with DoFn.MultiOutputReceiver#get(TupleTag).output(T), and review the @ProcessElement options mentioned in the issue. Done means the documentation explains when to prefer each multi-output approach and clarifies the available @ProcessElement options.

Written by the indexing model from the issue text.