Add "by" examples to aggregate function docs, plus a few more fixes

[philrz]

What's Changing

This adds group-by examples to all the aggregate function docs that were missing them.

As I was going through all the pages, I fixed a couple things along the way and added a bonus dcount() example.

Why

In a recent community Slack thread, a user needed some guidance on the proper use of by. He noted that by examples on each aggregate function pages may have helped him figure it out on his own, which makes sense.

Details

While by is mentioned in the summarize operator page (and I'm proposing more improvements there via #5216) I suspect that new users may be more likely to land on the individual aggregate function pages first since they may do searches or follow links to them after seeing references to them in programs as they usually appear without summarize.