clean up docs for v12

I reviewed docs like this:
git log -p remotes/origin/REL_11_STABLE..HEAD -- doc

And split some into separate patches, which may be useful at least for
reviewing.

I'm mailing now rather than after feature freeze to avoid duplicative work and
see if there's any issue.

Note, I also/already mailed this one separately:
|Clean up docs for log_statement_sample_rate..
https://www.postgresql.org/message-id/flat/20190328135918.GA27808%40telsasoft.com

Justin