This updates the documentation for changes originally made in commit
e89a71fb.

Patch by me, reviewed (but not entirely endorsed) by Amit Kapila.

Discussion: http://postgr.es/m/CA+Tgmoa+vupW8V_gBonz6hU7WwN2zJ=UTsVWCVB+rN6vaaXfZw@mail.gmail.com