Add comment about multiple output ports

The comments on the `topEntity` function of the starters and its `Synthesize` annotation already discuss some things worth knowing about these concepts. A new Clash user on Discord pointed out that they would have been helped by some comment about using a tuple to get multiple output ports:

_probeiuscorp wrote:_
> Setting up a new project was wonderfully easy! my first major point of confusion was the topEntity annotation. I would've found it very useful if the simple Stack template used a PortProduct for the output, as I saw that you could only output one thing and so took an hour trying to figure out how Clash solved those kinds of problems (skill issue?). I figured it while looking at the hackage docs for the Synthesize annotation and realized that the Clash mindset was to use the ordering in the tuples to associate with the port names. I guess I'd been expecting to have to do something with a named product in Haskell, like a derived typeclass on a record data type. I would've been helped a lot by a mention of that specifically in the comments on the simple Stack template.