doc review for v13

I reviewed docs for v13, like:
git log --cherry-pick origin/master...origin/REL_12_STABLE -p doc

I did something similar for v12 [0]. I've included portions of that here which
still seem lacking 12 months later (but I'm not intending to continue defending
each individual patch hunk).

I previously mailed separately about a few individual patches, some of which
have separate, ongoing discussion and aren't included here (incr sort, parallel
vacuum).

Justin

[0] https://www.postgresql.org/message-id/flat/20190709161256.GH22387%40telsasoft.com#56889b868e5886e36b90e9f5a1165186