Extended statistics commit 7b504eb2 did not include appropriate
documentation next to where we document regular planner statistics (I
ripped what was submitted before commit and then forgot to put it back),
and while later commit 2686ee1b added some material, it structurally
depended on what I had ripped out, so the end result wasn't proper.

Fix those problems by shuffling what was added by 2686ee1b and
including some additional material, so that now chapter 14 "Performance
Tips" now describes the types of multivariate statistics we currently
have, and chapter 68 "How the Planner Uses Statistics" shows some
examples.  The new text should be more in line with previous material,
in (hopefully) the appropriate depth.

While at it, fix a small bug in pg_statistic_ext docs: one column was
listed in the wrong spot.