Improve synchronous section

[HansOlsson]

Adds example, and moves one section that was out of place.
Closes #3783

[HansOlsson]

I believe I have now handle all of the comments.
Note the slightly wordier "Showing restrictions on super-sampling event clocks" instead of "Event clocks cannot be super-sampled", since (as the example shows) you can superSample something based on an event clock as long as it is effectively subSampling the signal.

Basically, people can interpret super-sampling to either mean using superSample operator, or super-sampling the original signal.

I'm not 100% happy with the text - but I think the example itself is so clear that people that it doesn't matter; and the text at least gives the context for the example, and the normative text describes it.

[henrikt-ma]

I don't know what's up with Github today, so I make a general comment here instead of attaching to a line where it belongs.

Wouldn't throwing in time like this (only for the first time mentioned in the sequence, for brevity) make it even better?

Here \lstinline!y! has the value 0 at time 0.0, 1 at 1.0, etc, \lstinline!ySub! has the value 0 at time 0.0, 4 at 4.0, 8 at 8.0 etc.

[henrikt-ma]

Besides this less important remark above, I think the only important thing remaining is a sentence to be moved into one of the examples, but I'm not familiar with the presentation of changes on Github today, so I could have missed something…

[henrikt-ma]

There is at least one place where the word time needs to be inserted. Apart from that I think it is time to merge and move on.

[henrikt-ma]

Time to move on!