Doc: update the "Using EXPLAIN" examples to match current code.

A lot of the examples in this section are quite ancient and no
longer match the system's current behavior.  I ran through them
and updated the sample output to match what I get today.

Notably, it seems that the tenk1 table got a few pages smaller back
around v12, so that a lot of the cost estimates for queries on that
table are a little different now.  (This also caused some examples
to surprisingly not match others, depending on when they were made.)

One mergejoin-based example no longer produces the same plan at all,
so I had to work around that, including switching to a different
example of using an enable_xxx setting.

I also changed a couple of examples that were relying on nonexistent
tables or indexes.  IMO the premise of this section is that the
examples should be more-or-less reproducible against the regression
database, so that these examples were unhelpful as given.  It's not
that hard to find a corresponding example with the tables we do have.

Some of the EXPLAIN ANALYZE examples were out of date in that we show
more numbers than we did at the time.

Possibly chapter 68 could use similar review, but I'm out of
energy for today.

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